Rodney B. Siegel
David F. DeSante
The Institute for Bird Populations
P.O. Box 1346
Point Reyes Station, CA 94956-1346
Last update: July 9, 1999
TABLE OF CONTENTS
Authors, recommended citation and acknowledgements
2. The Region
7. Literature Cited
Appendix 1: Species Accounts for the Landbird Avifauna of the Sierra Nevada
Appendix 2: Additional Sierra Species Not Included
in Appendix 1
Table 1. BBS population trend classification system
Table 2. Species trend classifications
Table 3. Statistically significant population trends
Table 4. Species that depend critically on montane meadows
Table 5. Species that are strongly associated with montane meadows
Table 6. Species that depend critically on late successional/old growth forest
Table 7. Species that are strongly associated with late successional/old growth forest
Table 8. Species that depend critically on oaks or oak woodlands
Table 9. Species that are strongly associated with oaks or oak woodlands
Table 10. Migratory status classification system
Figure 1. Meadow-dependent species— distribution of population trends
Figure 2. Meadow-dependent species— decreasing trends vs. increasing trends
Figure 3. LS/OG forest-dependent species— distribution of population trends
Figure 4. LS/OG forest-dependent species— decreasing trends vs. increasing trends 58
Figure 5. Oak-dependent species— distribution of population trends
Figure 6. Oak-dependent species— decreasing trends vs. increasing trends
Figure 7. Migratory status— distribution of population trends
Figure 8. Migratory status— decreasing trends vs. increasing trends
Figure 9. Comparison of MAPS productivity
indices for Yosemite National Park and Tahoe National Forest
David and Lucille Packard Foundation
National Fish and Wildlife Foundation
California Partners in Flight
The Institute for Bird Populations would like to thank J. Verner, J. Robinson, J. Steele, R. Stafani, K. Purcell, and G. Studinksi for sharing their ideas about Sierra bird conservation, the personnel of the Breeding Bird Survey for making available data and trend-estimating software, D. O'Grady and P. Nott for help with data analysis, and the numerous BBS observers and MAPS interns and contributors for their efforts in the field. We also thank the Point Reyes Bird Observatory for logistical and editorial support. This is contribution number 111 of The Institute for Bird Populations.
Siegel, R. B. and D. F. DeSante. 1999. Version 1.0. The draft avian conservation plan for the Sierra Nevada Bioregion: conservation priorities and strategies for safeguarding Sierra bird populations. Institute for Bird Populations report to California Partners in Flight.
This document is a draft avian conservation plan for the Sierra Nevada, produced for California Partners in Flight. The purpose of the draft plan is to summarize and analyze existing information on the status of Sierra bird populations, to identify major land management issues that may be threatening the security of those populations, and to suggest conservation actions to safeguard the populations and the habitats on which they depend.
Compared to other regions of California, the Sierra avifauna is still in relatively good condition, hosting only a handful of critically at-risk species (DeSante 1995). Evidence suggests, however, that many of the Sierra’s more common bird species may be declining. The Sierra Nevada was recently identified as one of 233 ecoregions whose biodiversity is outstanding on a global scale; unfortunately it was also identified as one of the 110 of those ecoregions considered critical or endangered (Olson & Dinerstein 1998). Problems facing the Sierra biota include a legacy of destructive land management practices reaching back to the Gold Rush, many current land management practices that still urgently need revising, and rapid human population growth, with its associated increases in land conversion and resource-use pressures.
Covering approximately 1/6 of the state of California, the Sierra Nevada’s diverse habitats are enormously important to the birds of California and, indeed, to a large portion of western North America’s Neotropical migratory birds. The best way to protect Sierra bird populations, those that are already seriously jeopardized as well as those that are not, is to proactively safeguard the habitats on which they depend. Although we limit our discussion in this report to the status and conservation of birds, most of our conservation recommendations are habitat-based, and would consequently benefit other jeopardized taxa as well.
Conservation issues in the Sierra Nevada are complex, and remedial actions, which at times may have to be based on ambiguous scientific information, will affect many diverse interest groups. This document should therefore be viewed as a starting point for discussion of avian conservation efforts in the Sierra. For a bioregional conservation agenda to be successful, many diverse voices must participate in setting conservation goals, and in formulating politically viable strategies to meet those goals.
2. THE REGION
This conservation plan, along with its accompanying species accounts, addresses the avifauna of the Sierra Nevada, from the upper foothills of the Sierra on the west slope to the base of the Sierra escarpment on the east slope. Elevational limits are necessarily vague, as montane conditions extend farther downslope in river canyons and on north-facing slopes than along ridges, south-facing slopes, and wide, flat river valleys. Roughly speaking, the lower elevational limit of montane conditions on the west slope averages 1,000'-1,500'. Excluded, then, are the flat to rolling grasslands and agricultural lands where the western flank of the Sierra joins the Great Central Valley, as well as the wide riparian riverbottoms, oak savannahs, lower elevation blue oak woodlands, and lower elevation chaparral covered slopes of the lower foothills.
The lower boundary of the area covered by this report is better defined on the more abrupt east slope than on the west slope and corresponds to the eastern base of the Sierra escarpment. In the south, the escarpment may be as low as 3,000' to 4,000', in the north it ranges from 4,000' to 5000', and in the central portion of the east slope it may be as high as 6,000' to 7,000'. Excluded on the east side are the grasslands, pasture lands, and riparian areas of the major valleys and basins east of the escarpment, including Owens Valley, Long Valley, Mono Basin, Bridgeport Valley, lower Walker and Truckee valleys, Sierra Valley, and Honey Lake Basin. Also excluded are the flat or gently sloping expanses of sagebrush, bitterbrush, pinyon pine, and juniper that characterize the basin and range portions of the Great Basin that abut the Sierra, as well as the desert scrub flats and slopes where the northern Mojave Desert borders the eastern flank of the Sierra. Finally, Jeffrey pine covered highlands that reach the Sierra at a few high passes between eastside valleys and basins, such as Deadman Pass where the east flank of the Sierra joins Glass Mountain, are also excluded.
In the north, the Sierra Nevada blends nearly imperceptibly into the southern extension of the Cascade Mountains in the Mt. Lassen area. Mt. Lassen and all points north of it are excluded, however, because Mt. Lassen is clearly of volcanic origin, in common with most of the higher peaks of the cascades. In the south, the Sierra curves southwestward to join the Tehachapi Mountains that in turn join the Transverse Ranges of southern California. We have arbitrarily chosen the southern limit of the Sierra to be the upper south-facing slopes of the South Fork of the Kern River and the upper slopes north of Walker Pass in Kern County. The ecosystem of the Valley of the South Fork of the Kern River and the neighboring lower canyon slopes including Walker Pass has a decidedly desert flavor, with many species characteristic of the California deserts.
This area corresponds roughly to the ‘Core Area’ defined by the Sierra Nevada Ecosystems Project (1996), but is slightly more restricted. In keeping with a true "Sierra" management plan, we have excluded areas that would have added substantially more species, habitats, and associated management issues, but are not genuinely montane in character (e.g., eastside alkaline lakes, such as Mono Lake; broad eastside and westside valleys, such as Sierra Valley, Walker Valley, Owen’s Valley; and the lower, Central Valley portions of most major west slope rivers, including the Feather, American, Tuolumne, San Joaquin and Kern).
State of Knowledge
DeSante (1995) synthesized information on the migratory status, distribution, abundance, demographics, and risks faced by each of the 146 landbird species (excluding diurnal raptors and gallinaceous species) that constitute the breeding avifauna of the Sierra. That synthesis, edited to reflect more recent population and demographic data, is reproduced here in Appendix 1.
The fundamental patterns of distribution and overall relative abundance have been described fairly well for many of the more common species in the Sierra. Much of the existing information, however, disproportionately describes the avifauna of the mid-elevation zone of the central Sierra, during the warmer months of the year.
The winter ecology of much of the Sierra’s birdlife is still poorly understood, including distributional questions (i.e., where do most of the Sierra’s Williamson’s Sapsuckers and Cassin’s Finches spend the winter?), life-history/demographic questions (i.e., how much of the annual mortality of Sierra resident birds is effected during winter months?) and resource management questions (i.e., how do various forest management practices affect avian community structure and nesting success?).
The birds of the rugged east slope of the Sierra are less well known than those of the west slope, and both the northern and southern Sierra appear to be less well studied than the central Sierra. Recent work has filled many gaps in our understanding of birds of the high country, but many basic aspects of the distribution and ecology of Sierra birds at lower elevations of the west slope remain unresolved. The chaparral of the lower-elevation slopes is extremely important to the overall populations of several rare or uncommon species, including Black-chinned Sparrow, Rufous-crowned Sparrow, Sage (Bell’s) Sparrow, and Lawrence’s Goldfinch. Reliable Breeding Bird Survey (BBS) trend data do not exist for any of these species, which could be undergoing serious declines, with little possibility of detection.
Nocturnal species represent another formidable gap in basic ecological information. Although the Spotted Owl and Great Gray Owl have, deservedly, received considerable attention and study, the most basic aspects of the distribution and ecology of the Long-eared Owl and most small owls in the Sierra remain almost a complete mystery. Common Nighthawk is the only nocturnal species for which reliable BBS trend data in the Sierra exist.
Although the Breeding Bird Survey provides the best long-term data available on population trends of Sierra birds, the historically low number of BBS routes conducted in the Sierra hampers conclusive assessments of the population trends of most species. Recent efforts within the Forest Service to add additional BBS routes throughout the Sierra will go a long way toward ameliorating this problem in the future, but overall avian monitoring efforts are still far from adequate to provide a thorough understanding of how resource management activities and other anthropogenic processes in the Sierra impact bird populations.
Compiling basic data on distribution and abundance constitutes only a first step toward understanding the population dynamics of Sierra birds.Adequate stewardship of Sierra bird populations requires increased efforts and new initiatives to elucidate:
a) habitat-specific population trends,
b) the primary demographic parameters that drive those population trends,
c) effects of management practices on primary demographic parameters, and
d) the status of numerous species that are too rare or locally distributed to be effectively surveyed by existing monitoring efforts.
Conservation Opportunities/Conservation Challenges
Now is a time of both unprecedented opportunity for conservation efforts in the Sierra, and increasingly difficult challenges. Resource managers and landowners throughout the Sierra are seeking more sustainable methods of resource extraction and are explicitly incorporating the conservation of biodiversity into resource management objectives. At the same time, population and recreational-use pressures are growing rapidly. The human population of the Sierra doubled between 1970 and 1990, resulting in extensive land conversion, and growth is expected to accelerate in the next decades (Duane 1996). Conserving the Sierra’s biodiversity in the context of rapid population growth will be a major challenge of the coming years.
Several recent and current largescale efforts to assess the ecological condition of the Sierra and to revise management practices have been initiated, including the Sierra Nevada Ecosystem Project’s (1996) four volume report to Congress, the USDA Forest Services’s ongoing Sierra Nevada Framework for Conservation and Collaboration and associated efforts to amend the management plans of each of the Sierra’s national forests, and the Sierra Nevada Forest Protection Campaign’s (1999) recent volume of Sierra-wide management recommendations. We hope this report will build on those efforts, by exploring the implications of various land management issues on the Sierra’s avifauna.
3. AVIFAUNA ANALYSIS
The extensive scope of this project requires that we take a ‘broad-brush’ approach to assessing the condition and needs of the Sierra avifauna. Readers interested in species-specific information are referred to the detailed species accounts in Appendix 1.
We analyzed population trend data from Breeding Bird Survey (BBS) routes located in the Sierra Nevada physiographic province. Droege (1990) and Peterjohn & Sauer (1993) provide detailed descriptions of BBS methodology and rationale. The BBS consists of a continent-wide array of roadside point-count transects, or routes. Each route is 39.4 km long, and comprises 50 3-minute point counts at 0.8 km intervals. Expert observers conduct point-counts once each year during the peak of the breeding season (June in the Sierra), recording numbers of every species detected within a 0.4 km radius.
BBS data have some important limitations. Reliable information is produced only for the more common species. Of the more than 150 bird species that constitute the complete Sierra’s breeding avifauna (Gaines 1992, DeSante 1995), only 77 were detected frequently enough on BBS routes between 1966 and 1996 to produce meaningful population trend estimates. The population status of rarer Sierra species, indeed, many of the species that are most likely to be in jeopardy, therefore cannot be evaluated using BBS data.
Additionally, BBS data are problematic because point counts are conducted exclusively at roadsides, which often include a large proportion of fragmented and edge habitats, and may not be representative of the larger habitat matrix. The result may be biases in the kinds of species that are detected, and in the number of individuals of some species counted (O’Connor 1992, DeSante & George 1994). Despite these shortcomings, BBS data provide the most extensive, long-term data set available on landbird population trends, and are a tremendously valuable resource for conservation planning.
We used the Interactive Route Regression Module provided on-line by the USGS Patuxent Wildlife Research Center to estimate population trends for the BBS Sierra Nevada physiographic province (stratum 66), from the inception of the BBS program in 1966 up through 1996. Unfortunately, the BBS has arbitrarily defined the Sierra Nevada physiographic province to encircle the north end of the Sacramento Valley, including the southern Cascades, and the Trinity Alps. Our analysis consequently includesfour routes (of a total 17 routes) that are actually located outside the Sierra Nevada as it is defined for this conservation plan. We can only hope that the inclusion of these data does not substantially bias trend calculations for the Sierra.
Province-wide trends were calculated as a weighted average of individual route trends, using the estimating equations estimator described in Link and Sauer (1994). The estimator model incorporates observer effects to prevent bias associated with changes in observer quality (Sauer et al. 1994).
BBS personnel suggest that a species must be detected on at least 14 different routes to provide enough data to reliably assess it’s regional populations trend. Because a maximum of only 17 routes were surveyed in the Sierra during the years under consideration, relatively few species met this threshold. To provide a meaningful framework for analyzing BBS population data, we consequently classified populations trends according to the system presented in Table 1. Species that were detected too infrequently to reach even the ‘increasing tendency’, ‘decreasing tendency’, or ‘stable tendency’ thresholds were excluded from the analysis.
We also used mark-recapture data gathered between 1992 and 1997 as part of the Monitoring Avian Productivity and Survivorship (MAPS) program (DeSante et al. 1995, 1998, Burton and DeSante 1998). The MAPS program collects bird-banding data from over 500 stations across the North American continent, adopting a 'constant-effort’ mist-netting method to index productivity and using mark-recapture techniques to estimate survivorship of landbirds. Bird-banding teams at each station run approximately 10 mist nets within the central 8 ha of a 20-ha plot for six hours following sunrise. Each station is visited on one day within each of 7-8 sequential ten-day periods throughout the breeding season (May 21 or May 31 [depending on altitude] to August 8 in the Sierra). Remote sensing of habitat patterns in the vicinity of each station in conjunction with on-ground spatial habitat assessment and local climate data forms the basis of a geographical information system (GIS) for the MAPS program (MAPSIS). This system is used to identify habitat patterns (at a variety of spatial scales) that are associated with declining (or increasing) population trends caused by low (or high) productivity or survivorship for target species.
Seventy seven species were detected frequently enough to allow the calculation of Breeding Bird Survey population trends estimates (Appendix 1). The trend classifications of these 77 species are presented in Table 2. Overall, 40 species (51.9%) exhibit negative trends, 18 species (23.4%) exhibit positive trends, and 19 species (24.7%) exhibit stable trends. Of the 58 species exhibiting either positive or negative trends in the Sierra, significantly more appear to be declining than would be expected due to chance alone (Binomial Test, P < 0.05). Table 3 lists only those species whose Sierra population trends are statistically significant, at various thresholds. Overall, 18 species show significantly decreasing trends, compared to only 4 species with significantly increasing trends; again, this preponderance of declining species differs significantly from the distribution expected by chance (Binomial Test, P < 0.01).
Dependence on particular habitats
Species dependent on a few critical habitats within the Sierra appear disproportionately likely to exhibit decreasing trends. These habitats include montane meadows, non-meadow riparian habitat, late successional/old growth forest, and oak woodland.
Montane meadows -- Based on the species accounts in Appendix 1, we compiled lists of a) species that critically depend on Sierra montane meadows for at least a portion of their life cycle (Table 4) and b) species that are strongly associated with montane meadows, but do not necessarily critically depend upon them (Table 5). Of the 37 species on the combined lists, 13 are inadequately sampled by the BBS to allow the calculation of a population trend. Among those 13 are two California Endangered Species (Willow Flycatcher and Great Gray Owl) and a California Bird Species of Special Concern (Vaux’s Swift) (Comrack 1992). Of the 24 species with adequate BBS data to calculate population trends, 6 are stable, 14 are decreasing, and 4 are increasing (Fig. 1); the preponderance of decreasing species is statistically significant (Binomial Test, P < 0.05;
Non-meadow riparian habitat -- Thirteen of the meadow-dependent species in the Sierra are also dependent on non-meadow riparian habitat. They include Long-eared Owl, Belted Kingfisher, Black Phoebe, Warbling Vireo, Tree Swallow, Northern Rough-winged Swallow, House Wren, Swainson’s Thrush, Yellow Warbler, MacGillivray’s Warbler, Wilson’s Warbler, Song Sparrow, and Lazuli Bunting. An additional 16 non-meadow species are dependent on non-meadow riparian habitat in the Sierra, although many of these occur primarily in the lower foothills of the Sierra. These 16 species include Great Blue Heron, Wood Duck, Harlequin Duck, Common Merganser, Red-shouldered Hawk, Killdeer, Spotted Sandpiper, Black-chinned Hummingbird, Downy Woodpecker, Pacific-slope Flycatcher, Winter Wren, American Dipper, Yellow-breasted Chat, Black-headed Grosbeak, Blue Grosbeak, and American Goldfinch. Of the 14 of these combined 29 species with adequate data to calculate population trends, 5 are stable, 6 are decreasing, and 3 are increasing. We refer the reader to Riparian Habitat Joint Venture (1998) for an extensive analysis of the status of riparian-dependent birds throughout California.
Late successional/old growth forest -- We used data from the California Wildlife Habitat Relationships Database system (California Department of Fish and Game 1994) to compile lists of a) species whose population viability in the Sierra requires late successional/old growth (LS/OG) forest habitats (Table 6), and b) species that use LS/OG habitat, but whose populations do not critically depend on it (Table 7). The combined lists comprise 34 species, 14 of which are inadequately sampled by the BBS to calculate population trends. Once again, some of the most high-risk species (i.e., Northern Goshawk and Spotted Owl) are those that are lacking in BBS data. Of the 20 species with calculable population trends, 5 are stable, 4 are increasing, and 11 are decreasing (Figure 3). The ratio of decreasing to increasing species is skewed heavily toward decreasing species, but does not quite reach the threshold of statistical significance (Binomial Test, P > 0.05, Fig. 4).
Oak woodlands -- We used the species accounts
in Appendix 1 to compile lists of a) species that critically depend on
the Sierra’s oak woodlands (Table 8) and
b) species that are strongly associated with oak woodland, but do not necessarily
depend critically upon it (Table 9). Of
the 30 species on the combined lists, 11 are insufficiently sampled by
the BBS to produce reliable population trends. Three of the remaining species
have population trends that are stable, 4 have increasing trends, and 13
have decreasing trends (Fig. 5). The ratio
of decreasing trends to increasing trends departs significantly from that
expected by chance (Binomial Test,
P < 0.05; Fig.
Problems in the Sierra Nevada or on the wintering grounds?
Observed declines of Sierra bird populations could be due to diminished productivity and/or survival in the Sierra, or to reduced survival on the wintering grounds or along migratory routes, at least for migratory species. To help distinguish between these two possibilities, we assessed migratory status as a predictor of population trend direction.
DeSante (1995; Appendix 1) classified the migratory status of each Sierra bird species as either resident (R), resident/short-distance migrant (R-SDM), short-distance migrant (SDM), short-distance migrant/Neotropical migrant (SD-NTM), or Neotropical migrant (NTM), according to the system in Table 10. Within each migratory status classification, we tallied the number of species (of the 77 species with calculable trends) that were increasing (including everything from ‘increasing tendency’ to ‘definitely increasing’), stable (including everything from ‘stable tendency’ to definitely stable’), or decreasing (including everything from ‘decreasing tendency’ to ‘definitely decreasing’). If problems driving the apparent decreases in Sierra bird populations were primarily encountered on the wintering grounds of migratory species, we would expect strictly resident birds to include relatively fewer species with decreasing trends than would migratory birds. Figure 7 shows that this is not the case; over 61% of strictly resident species with calculable trends show decreases, compared to only about 42% of Neotropical migrants. If anything, resident birds appear to be faring worse than Neotropical migrants, suggesting that pervasive ecological problems may be affecting productivity and/or survival rates within the Sierra. Patterns of population trends exhibited by the three classes of short-distance migrants are difficult to interpret. Resident/short-distance migrants include the lowest proportion of decreasing population trends (33.3%), while short-distance migrants account for the highest proportion of decreasing trends (65.0 %). We obtained similar results when we considered only the 50 species with ‘definite’, ‘likely’, or ‘possibly’ trend classifications, as well as to only the 26 species with ‘definite’ or ‘likely’ trend classifications, and to only the 16 species with ‘definite’ trend classifications.
In order to create categories with adequate sample sizes for performing binomial tests, we further aggregated migratory status classifications into three broad categories: 1) resident or resident/short-distance migrant (R/R-SDM), 2) short-distance migrant or short-distance/Neotropical migrant (SDM/SD-NTM), and 3) Neotropical migrant (NTM). We then tallied the species with decreasing and increasing Sierra-wide trends in each category (Fig. 8), again assuming that non-stable trends should be equally distributed between decreasing and increasing species.
Among both R/R-SDM and NTM species, this was indeed the case; more species exhibited decreasing than increasing trends, but the distribution was not significantly different than that expected due to chance (Binomial Tests, P > 0.05). SDM/SD-NTM species, however, were dramatically more likely to be decreasing than increasing (Binomial Test, P < 0.005). Again, similar results (significantly more decreases than increases for SDM/SD-NTM species; more, but not significantly more, decreases than increases for R/R-SDM and NTM species) were obtained when we considered only ‘definitely’, ‘likely’, or ‘possibly’ trends; ‘definitely’ and ‘likely’ trends only; or ‘definitely’ trends only. Although much concern has been raised about declining populations of Neotropical migrants throughout North America, BBS trend data suggest Neotropical migrants are faring no worse in the Sierra than other species. Rather, short-distance migrants seem to be at the greatest risk. These results agree with Hutto (1988), who questioned the decline of Neotropical migrants wintering in western and southern Mexico, and with DeSante and George (1994) who found that Neotropical migrants generally showed fewer and smaller decreasing trends than short-distance migrants throughout the western United States.
These results should not be interpreted as indicating that problems do not exist among Neotropical migrants, nor that tropical deforestation is not a problem for some migratory Sierra birds, but merely that gross generalizations regarding massive declines of Neotropical migrants in western North America in general, and the Sierra Nevada in particular, may be unfounded. More importantly, more attention should be focused on problems the Sierra’s short-distance migrants may be encountering on their wintering grounds in southern California Arizona, and northern Mexico. Finally, the relatively high proportion of decreasing population trends in all categories, particularly among strictly resident birds, suggests that pervasive productivity and/or survivorship problems may exist within and throughout the Sierra.
Can decreasing population trends be linked to land management practices?
The design of the BBS monitoring system makes correlating population trends with land management practices virtually impossible. Point count transects that are 34.9 km long generally pass through lands that include widely differing habitat types, let alone management regimes. Once data from the various points along each transect are combined (which is necessary to amass an adequate sample size for trend detection), adequate resolution for correlating abundance data with land management practices (or even habitat types) is necessarily lost.
Standardized, constant-effort mist-netting data from the MAPS program may provide bird population indices for more circumscribed areas than BBS routes, and therefore provide the potential for linking specific land management regimes to their consequences for local bird populations. Additionally, MAPS data provide information on primary demographic parameters, so that observed population changes can be attributed to changes in either productivity, survival, or both. We used data from 12 Sierra Nevada MAPS stations operated between 1992 and 1997 to investigate whether observed population declines may be linked to Sierra land management practices. Five of the stations were located in Yosemite National Park, and seven were located on or adjacent to the Tahoe National Forest. Stations in the two areas, which are were mostly located along the edges of montane meadows, spanned roughly equivalent elevational gradients. Station elevation averaged 1,804 m in Yosemite (minimum elevation = 1,311 m; maximum = 2,402 m) and 1,887 m in the Tahoe area (minimum elevation = 1,494 m; maximum = 2,042 m).
We compared productivity indices (per cent of the catch comprised of juveniles) by combining capture data within each of the two sets of stations. Adequate data existed to calculate productivity at both sets of stations for 22 species (Fig. 9). Sixteen of the 22 species (73%) had higher productivity indices in Yosemite than at Tahoe; of the 16 species whose productivity indices from the two sets of stations differed by 10% or more, 12 (75%) had higher productivity in Yosemite. Of considerable interest is the a fact that 14 out of 16 species (88%) with higher productivity indices in Yosemite National Park than on the Tahoe National Forest are meadow or late successional/old growth dependent species, while only 3 out of 6 species (50%) with higher productivity on the Tahoe than in Yosemite are meadow or LS/OG dependent species.
We believe that this preponderance of higher productivity indices at the Yosemite stations may reflect differences resulting from land management regimes at and around the two sets of stations. Despite decades of fire suppression efforts and some historical grazing, forest stands and meadows in the Sierra’s national parks are still probably more similar to their pre-European settlement conditions than forest stands and meadows in the national forests, which have often been subject to heavy grazing and logging pressures. While inconclusive, this comparison of indices strongly suggests a problem of diminished productivity on the historically more heavily managed lands of the Tahoe National Forest.
4. PRIORITY HABITATS FOR CONSERVATION
Based on the species accounts in Appendix 1, our avifauna analysis, and other information, we identified the following habitats as top conservation priorities within the Sierra Nevada: montane meadows, non-meadow riparian habitat, late successional/old growth forest and oak woodland. Although montane meadows are sometimes included with streamside vegetation in more general discussions of ‘riparian’ habitat, we believe that the critical role that Sierra montane meadows play in supporting numerous Sierra bird species (many of which are not normally associated with other types of riparian habitat) merits treating them as a separate habitat category.
In the following sections we identify and briefly summarize major land
management issues affecting each of the top priority habitats. The summaries
are not intended to be exhaustive; fully addressing the complexity of each
issue, and even thoroughly summarizing the relevant literature is beyond
the scope of this project. Rather, our more limited objectives are to identify
those land management issues that are most likely to affect the security
of Sierra bird populations, and to point the reader toward more detailed
discussions of the issues.
PRIORITY HABITAT 1: MONTANE MEADOWS
Montane meadows are a distinct type of riparian plant community, generally dominated by sedges, but also including rushes, grasses and forbs. Whether or not surface water is present, high ground water excludes most plant species (Kattelman and Embury 1996), even in meadows that are generally classified as ‘dry’ (Whitney 1979). Sierra meadows range in size from just a few square meters to several square kilometers (Allen 1987), and most commonly occupy glaciated subalpine zone basins, although smaller numbers of meadows are found all the way down to 1,200 m in the northern part of the mountain range, and 1,800 m in the south.
Montane meadow habitat is extremely important to the Sierra avifauna. Not only do numerous species depend on montane meadows for breeding habitat, but meadows also serve as important supplemental habitat for many species that breed in other habitats; examples include Red-breasted Sapsucker, which utilizes willows in montane meadows for a steady supply of sap during the breeding season, and several finch species which require a daily water supply (DeSante 1995). Additionally, montane meadows provide critical molting and pre-migration staging areas for juveniles and adults of a broad array of Sierra landbird species, many of which also do not actually use meadow habitat for breeding. For some of these species, such as Orange-crowned and Nashville Warblers, montane meadows in mid-summer may be the single most critical Sierra habitat requirement (DeSante 1995). Finally, the population densities of many forest-inhabiting species are often highest near meadow edges, even if the birds rarely or never actually venture into the meadows (DeSante 1995).
Historic and current human activities, particularly livestock grazing, have compromised the viability of meadow habitat for birds in many parts of the Sierra Nevada. Severe overgrazing between the late 1800s and about 1930 heavily impacted Sierra meadows, resulting in accelerated erosion and massive gullying (reviewed in Menke et al 1996, Kattelman and Embury 1996). Although the worst abuses were halted, Sierra meadows were still heavily grazed up until the 1970s, when the Forest Service began to take a more ecologically oriented approach to range management. Although conditions have subsequently improved, the continuing presence of large livestock herds in many areas continues to impact meadow ecosystems today.
California’s Endangered Species list includes two meadow-dependent birds, Willow Flycatcher and Great Gray Owl, and cattle grazing has been implicated in the decline of each (Serena 1982, Harris et al. 1987, Gaines 1988, Harris et al. 1988, Ohmart 1994, Graber 1996). Willow Flycatchers avoid nesting in the lowermost foliage of willows that have been denuded by cattle grazing (Duff 1979, Gaines 1988, Fowler et al. 1991). Great Gray Owls depend on meadow vole populations, which livestock grazing can reduce (USFWS 1980, Hanley and Page 1982, Kauffman et al. 1982, Winter 1986, Kie 1991, Greene 1995). Additionally, Great Gray Owls may be excluded from foraging in grazed meadows by Great Horned Owls, which apparently prefer grazed meadows (Gaines 1992).
More generally, improper grazing practices have a variety of effects on meadows that are ultimately deleterious to many Sierra bird species. In addition to trampling nests (Skovlin 1984), livestock grazing can reduce herbaceous vegetation cover, making habitat unsuitable for many riparian birds that are fairly sensitive to changes in complexity and density of vegetation structure (Dobkin 1994). Moreover, many meadow-associated bird species depend upon invertebrate prey that in turn require herbaceous plants for sustenance. Reduced herbaceous plant cover consequently may result in reduced food resources for birds. Heavy grazing over many years can also prevent shrub and tree regeneration, eliminating essential nesting and foraging habitat (Skovlin 1984).
Improper grazing practices reduce vegetation cover and alter vegetation composition (Holechek et al. 1989). They can also cause soil compaction and damage the banks of streams, resulting in increased channelization and a general drying out of meadows, and ultimately, hasten forest encroachment (Odion et al. 1990, Ohmart 1994, Kattelmann and Embury 1996, Menke et al. 1996). Such problems are widespread throughout Sierra meadows(DeBenedetti and Parsons 1979, Ratliff 1985, Hagberg 1995, Moyle 1996).
Finally, the presence of cattle in montane meadows may attract cowbirds, which parasitize nests of many bird species in the surrounding forest (Verner and Ritter 1983, Rothstein et al.1980, Rothstein et al. 1984, Laymon 1987, Graber 1996). Although BBS data indicate that Brown-headed Cowbirds may be declining in the Sierra, nest parasitism is still an important issue, especially for the most vulnerable species, which are generally riparian-dependent(Airola 1986).
For all of these reasons, poorly managed grazing in riparian areas can reduce nesting densities of many bird species (reviewed by Fleischner 1994, Saab et al. 1995), particularly of habitat specialists such as Willow Flycatcher, Lincoln’s Sparrow and White-crowned Sparrow (Knopf et al. 1988).
Other human activities in the surrounding watershed can also contribute to the gradual drying out of meadows. Examples include road construction (often related to forestry activities) and deforestation associated with logging, both of which can result in increased water runoff and consequent downstream channel incision. As meadow stream channels become incised, the surrounding water table is lowered, and flood events capable of inundating the surrounding meadow become increasingly rare. Substantial changes in vegetation, including loss of woody riparian vegetation (i.e., willows and alders), forest encroachment, and sweeping changes in graminoid community composition can then result.
PRIORITY HABITAT 2: NON-MEADOW RIPARIAN HABITAT
The loss and degradation of riparian habitats have been implicated as key factors in population declines of western North American landbirds (Terborgh 1989, DeSante and George 1994, Ohmart 1994). As in other regions, riparian zones in the Sierra are crucial to the numerous bird species that utilize them for breeding, to many additional species which depend on them as migration stopover areas, and even to many species of upland-dwelling birds, whose populations densities are often elevated adjacent to riparian areas (Carothers 1977). Riparian areas have been identified as the single most critical habitat for avian conservation across California (Miller 1951, Manley and Davidson 1993, Riparian Habitat Joint Venture 1998). Because of a) the critical importance of riparian habitat to numerous Sierra species, and b) the extensive anthropogenic modification of Sierra riparian areas, riparian habitat should be considered a high-priority habitat within the Sierra Nevada.
Because of it’s narrow, linear configuration, riparian habitat in the Sierra occupies a very small proportion of the overall landscape— estimates vary considerably depending on the particular criteria used for delineation, but all sources place the value at well under 5% (reviewed in Kattelman and Embury 1996). Davis and Stoms (1996) estimate that riparian forest and riparian scrub cover a total of approximately 59 km2 and 119 km2, respectively, in the Sierra (as delineated by the Sierra Nevada Ecosystems Project).
Surprisingly little attention has been paid to Sierra riparian ecology in the scientific literature (reviewed in Kondolf et al. 1996), with even fewer studies directly addressing the Sierra’s riparian avifauna. This is unfortunate because riparian corridors throughout the Sierra are badly in need of conservation and restoration measures. A recent aerial survey indicated that extensive loss and fragmentation of riparian vegetation is common along most Sierra riparian corridors, especially at lower elevations (Kattelman and Embury 1996).
Kondolf et al. (1996) and Kattelman and Embury (1996) exhaustively review major human impacts on riparian areas in the Sierra, and conclude that the most prevalent present-day causes of riparian habitat loss and fragmentation are road and railroad crossings, timber harvesting, clearing of private lots/urbanization, water diversion for hydroelectric generation or irrigation, livestock grazing and inundation for reservoirs.
The California Partners in Flight Riparian Habitat Conservation Plan (Riparian Habitat Joint Venture 1998) provides an extensive discussion of important riparian habitat conservation issues, and should be referred to for more detailed information.
PRIORITY HABITAT 3: LATE SUCCESSIONAL/OLD-GROWTH FOREST
The aerial extent of late successional/old growth (LS/OG) forest, as well as overall structural complexity throughout the low- to mid-elevation forest belts, has been greatly reduced by past and present logging practices and human-altered fire regimes (Franklin and Fites-Kaufmann 1996). In their extensive assessment of the status of LS/OG forest in the Sierra, Franklin and Fites-Kaufmann reach several major conclusions, reproduced below:
1. There is relatively little high-quality late-successional forest remaining in the Sierra Nevada, particularly in the commercial forest zones.
2. Commercially important forest types— such as the westside mixed-conifer and eastside pine forests— are most deficient in high-quality late-successional forest relative to their potential and to presettlement conditions.
3. Key structural features of the late-successional forests— such as large diameter trees, snags, and logs— are generally at low levels in the forests of the Sierra Nevada.
4. On the positive side, the forest cover of the Sierra Nevada is relatively intact and most forest stands have sufficient structural complexity to provide for at least low levels of late-successional forest function...[However] while forest continuity is high in the Sierra Nevada, as noted above, the forest structure has been greatly simplified relative to presettlement conditions so that the forests do not provide the same level of wildlife habitat and other ecological functions characteristic of high quality LS/OG forests.
5. National parks provide the major concentrations of high-quality late successional forests, especially at the landscape level, and on a percentage basis, have about twice as much highly rated forest [LS/OG characteristics] as adjacent national forests.
6. Much of the highly-ranked late-successional forest on national forest lands is unreserved and potentially available for harvest.
These changes in LS/OG habitat availability, particularly the loss of LS/OG forest characteristics such as large trees, abundant snags, and large downed logs, have placed many high-profile LS/OG dependent bird species such as Spotted Owl, Northern Goshawk and Great Gray Owl at risk, along with several LS/OG dependent mammals, including fisher, American marten, Sierra Nevada red fox, and wolverine (Verner et al. 1992, Powell and Zielinski 1994, Graber 1996). Effects on numerous lower-profile species are poorly known.
Reduction in the aerial extent of LS/OG forest in the Sierra reflects pervasive changes in forest structure throughout the Sierra since the mid- 19th century. Timber harvest practices and fire suppression have reduced the frequency of low-intensity fires, reduced the number of large trees, increased the density of smaller, understory trees, and possibly reduced the extent of shrub cover and the density of snags (Weaver 1974, Vankat and Major 1978, Parsons and DeBenedetti 1979 McKelvey and Johnston 1992, Andrews 1994, Hejl 1994, Chang 1996, Franklin and Fites-Kaufmann 1996). Suppression of surface fires, in particular, also affects forest community composition, favoring the recruitment of white fir over pines and black oak (Agee et al. 1978, Husari 1980, Chang 1996). Gradual conversion to white fir-dominated stands is consequently in progress across much of the Sierra (Parsons and DeBenedetti 1979, Bonnicksen and Stone 1982, van Wagtendonk 1985, Weatherspoon et al. 1992). All these changes in forest structure and composition have surely had far-reaching effects on avian community composition (Beedy 1982, Raphael and White 1984, Raphael et al. 1987, Hejl 1994), although adequate data for reconstructing those effects are lacking.
Another, more indirect effect of fire suppression practices may be a gradual loss of habitat diversity throughout Sierra forests. The accumulation of downed logs and other fuel that has resulted from fire suppression, along with the increasingly dense understory, have made large, high-severity crown fires more common (Andrews 1994). Such large, intense fires typically kill vegetation over broad areas, ultimately increasing homogeneity and patch sizes within affected forests (Weatherspoon et al. 1992, Andrews 1994, Skinner and Chang 1996). The long-term result is a less diverse forest, capable of supporting a less diverse avifauna.
PRIORITY HABITAT 4: OAK WOODLANDS
The human population of the Sierra Nevada doubled between 1970 and 1990, with 40% of that growth occurring in the foothill zones of Nevada, Placer and El Dorado Counties (Duane 1996). Human population is predicted to increase three-fold between 1990 and 2040, with the land area developed for human settlement increasing four-fold (Duane 1996). This rapid land conversion presents an obvious threat to the whole suite of species dependent on foothill blue oak woodlands.
Equally important to numerous bird species for nesting and roosting (as well as acorn-dependent species) are the oaks of the lower conifer zone (black oak, canyon oak, interior live oak, and tanoak). Although urbanization is not as much of a threat in this zone as in the blue oak elevational belt, both blue and black oaks have shown alarmingly poor recruitment throughout the last half century, possibly as a result of fire suppression (McClaran and Bartolome 1989, Chang 1996). Patterns of oak distribution and abundance prior to European settlement are believed to have been actively maintained through extensive use of low-intensity fires by native Americans (Anderson 1993, Anderson and Moratto 1996). Without a dramatic change in fire regimes, it has been suggested that black oak habitat in the Sierra will eventually occupy only a small fraction of its current distribution (McDonald 1990), with dire consequences for oak-dependent birds.
5. CONSERVATION RECOMMENDATIONS
Objective 1. Safeguard existing high-quality meadow habitat throughout the Sierra.
Recommendation 1-1. Use a standardized methodology to identify meadows throughout the Sierra that are particularly valuable to breeding, dispersing, and migrating birds.
Initial lists of a tractable number (30-50?) candidate meadows for top-priority conservation status within each of the Sierra’s national forests and national parks, as well as additional meadows on privately held lands, should be produced using GIS and other existing data. Depending on available data, criteria for candidacy could include elevation, size, presence and extent of surface water, presence and size of willow thickets, per cent cover of the surrounding forest, or remoteness from other candidate meadows. Current Forest Service efforts to model potential Willow Flycatcher habitat may produce additional criteria.
Once lists of candidate meadows have been developed, a standardized, rapid field methodology should be implemented to survey each candidate meadow. The Institute for Bird Populations has recently developed such a protocol, and has implemented it in portions of the southern Sierra.
Several different criteria should then be considered in selecting surveyed meadows for top-priority conservation status, including:
a) presence of species of management concern,
b) particularly high species richness and/or relative abundance of breeding bird species,
c) high concentrations of dispersing juveniles (these data can be standardized with respect to seasonal timing and elevation by comparison with analogous data from Sierra MAPS stations operated each summer for many years at varying elevations), and
d) geographical remoteness from other high-quality meadows.
Recommendation 1- 2. Confer formal protection status on high-priority meadows.
A proactive, Sierra-wide strategy for safeguarding meadow habitat is necessary to ensure the long-term viability of populations of many vulnerable meadow-dependent Sierra bird species. We suggest conferring Important Bird Area (IBA) status on a Sierra-wide network of meadows, including the 12 to 18 highest priority meadows within each administrative unit (national park or national forest) and additional meadows on privately held lands. All these meadows would be included in a Sierra Meadows Important Bird Area (IBA).
The IBA approach has been found to be an effective habitat management and conservation tool, and has been widely adopted around the world. The International Council for Bird Preservation (now Birdlife International) started the program in the mid-1980s as a comprehensive approach to encouraging conservation of sites that provide essential habitat to vulnerable or endemic species, or unusually large concentrations of birds. The program has been enormously successful, with over 3,000 IBA sites designated worldwide, and numerous sites throughout the United States. The IBA strategy has been explicitly endorsed by Partners in Flight, and was recently codified into New York state law.
IBA designation would not necessarily exclude management activities such as livestock grazing, but it would require that any such activities be consistent with the IBA’s primary goal of maintaining high-quality bird habitat. The IBA designation would carry no legal weight, but would serve to remind land managers and resource users of the meadows’ critical importance to Sierra birds.
Members of the US Forest Service’s Sierra Nevada Framework for Conservation and Collaboration EIS team have recently expressed an interest in incorporating a formal meadow reserve network into the revised management plans for the Sierra’s national forests. Such an approach would likely have even stronger impacts on meadow management practices than would an IBA system, and should be encouraged. Regardless of which system of protection is adopted, designated meadows should be managed explicitly to maintain populations of meadow-dependent birds.
Recommendation 1-3. Use existing information to develop and implement management prescriptions for protected meadows.
A ‘toolbox’ of possible strategies for reducing the deleterious effects of various human activities on montane meadow ecosystems should be produced by searching and synthesizing the vast published literature and consulting with range management experts. Strategies should focus primarily on lessening the negative impacts of livestock grazing, but the effects of all relevant land management practices should also be reviewed. A resulting summary of meadow management alternatives could form the basis of the management prescriptions for the designated IBA meadows, and would also be extremely useful to managers of montane meadow habitat throughout western North America. The summary should be made widely available to public and private land managers.
Subsequent to field assessments and selection of IBA meadows, an initial set of management prescriptions should be formulated for each of the designated IBA meadows, in conjunction with the appropriate land holders/land managers. Prescriptions should be specific to each meadow and should be based on the ‘toolbox’ of options described above. Most importantly, prescriptions must be acceptable to land holders/land managers, so that they will actually be implemented.
Recommendation 1-4. Monitor the effectiveness of the management prescriptions implemented on the protected meadows and, if necessary, modify the prescriptions and implement them in an adaptive management framework.
It is crucial that the management prescriptions implemented on the protected meadows can be demonstrated to be effective at safeguarding the avian resources of the meadow. This is important because the management prescriptions implemented will likely tend to reduce the extent of certain other activities, such as livestock grazing, extraction of wood products, and construction of roads, on and immediately adjacent to the protected meadows. This can be accomplished by monitoring the populations of breeding and dispersing birds in the protected meadows and comparing these populations to those in paired control meadows on which management prescriptions are not implemented. For optimal results, monitoring and management should be linked within a research framework; this is the essence of adaptive management (Holling 1978, Noss and Cooperrider 1994).
Recommendation 1-5. Focus research efforts on the effects of different livestock grazing practices on the abundance, diversity, and nesting productivity of meadow-dependent birds. Although adequate scientific information currently exist to formulate preliminary meadow management guidelines, hypothesis-driven research on the effects of specific land management practices on meadow avifauna are badly needed. In particular, research should focus on ways of reducing potentially harmful effects of livestock grazing and other management practices.
Objective 2. Restore degraded meadows to enhance their value for breeding, dispersing, and migrating birds.
Recommendation 2-1. Encourage landholders (public and private) and resource users to incorporate avian habitat needs into management practices on all meadows, not just those with formal protection status.
Management guidelines based on the best available information (see Recommendation
1-3) should be made widely available to Sierra land managers (both public and private). Concise, easily accessible information, combined with a vigorous outreach campaign, should encourage land managers of meadows not included in the IBA to factor the needs of Sierra birds into their management decisions as well. Because meadows are considered more responsive to changes in management and reductions in grazing intensity than any other type of range ecosystem (Menke et al. 1996), even relatively minor changes in management practices may produce important benefits for meadow-dependent birds.
Recommendation 2-2. Where feasible, promote active restoration of meadows, including re-vegetation and restoration of natural hydrologic processes, in an adaptive management framework.
Recent advances in stream restoration suggest that even deeply incised meadow streams can be successfully altered to restore natural flooding regimes to unnaturally dried-out meadows (Jim Wilcox, pers. comm.). This is especially true where stream incision and associated drying of meadows primarily reflect the effects of past, rather than current land management practices. Meadow restoration projects, incorporating hydrologic as well as vegetative restoration, should be encouraged on public and private lands throughout the Sierra. To maximize the benefits from revegetation and restoration efforts, their effectiveness must be monitored and the protocols and techniques modified in an adaptive management framework. A twofold approach to such monitoring is useful and involves: (1) monitoring breeding and dispersing bird populations in such meadows before and after revegetation and restoration efforts; and (2) monitoring revegetated and restored meadows and paired control (unrestored) meadows.
NON-MEADOW RIPARIAN HABITAT
The 14 comprehensive riparian conservation objectives developed by the Riparian Habitat Joint Venture (1998) provide an excellent framework for prioritizing riparian bird conservation efforts within the Sierra, as well as throughout California. The reader is referred to that document for conservation objectives and recommendations.
Objective 3. Safeguard existing high-quality LS/OG habitat throughout the Sierra.
Recommendation 3-1. Create a Sierra-wide LS/OG reserve network, to ensure the long-term maintenance of habitat for LS/OG-dependent birds.
The Sierra Nevada Forest Protection Campaign (Britting et al. 1999) recently called for the establishment of a an LS/OG reserve network, using the regions identified as Areas of Late Successional Emphasis in the SNEP report. This LS/OG conservation approach is one of two reserve design strategies endorsed by the SNEP Working Group on Late-Successional Conservation Strategies (Franklin et al. 1997). Such a reserve network would be managed for "the conservation and restoration of high quality LS/OG forests and associated ecosystem processes in those forests most strongly affected by the commercial logging and fire suppression practices of the past 150 years: westside mixed conifer, westside pine, red fir, eastside mixed conifer, and eastside pine" (Britting et al. 1999). We believe this strategy provides a workable means of safeguarding essential habitat for LS/OG-dependent birds. Again, as with the meadow reserve system, the results of management prescriptions implemented on LS/OG reserves must be monitored and modified in an adaptive management framework.
Recommendation 3-2. Study and revise fire and other management practices to help restore the Sierra’s natural level of forest diversity, and to promote a gradual increase in the aerial extent of LS/OG forest.
Fire regimes should be as similar to naturally occurring patterns as possible, in order to promote the long-term development of LS/OG forest, and to maintain historic levels of forest diversity and patchiness. In many parts of the Sierra, large fires present a substantial risk to humans and their property, and therefore are neither practical nor politically feasible. Moreover, in many areas the size and extent of fuel loads that have resulted from decades of fire suppression may prohibit the use of controlled fire regimes. Mechanically manipulating vegetation to mimic the forest structures created by fire is a potential solution; studies of its effects on forest ecology and avian community composition and nesting success are urgently needed.
Objective 4. Continue and improve efforts to monitor the population status of LS/OG dependent species.
Recommendation 4-1. Focus research efforts on the status of LS/OG-dependent species not adequately monitored by the BBS approach.
Much of the Sierra’s remaining LS/OG habitat remains in areas with limited road access, and is consequently poorly sampled by BBS roadside transects. Indeed, nearly 60% of LS/OG-associated species are detected too infrequently during BBS surveys to allow calculation of reliable BBS trends (Tables 7 and 8). A handful of these species are known to be declining and/or very localized in their distribution, and are already under intensive study (e.g. Spotted Owl, Northern Goshawk), but the Sierra-wide status of others is relatively unknown. Monitoring and research efforts focusing specifically on LS/OG-dependent avifauna are urgently needed.
Objective 5. Halt the rapid destruction of oak woodlands in the Sierra foothills.
Recommendation 5-1. Mount a vigorous public outreach campaign to insert a concern for oak woodlands preservation and the habitat needs of oak-dependent wildlife into the growth plans of Sierra foothill communities.
Unlike most conservation problems in the conifer forest belt, solutions to problems in oak woodlands depend primarily on actions on private rather than public lands; a relatively meager proportion of the Sierra’s oak woodlands are contained in national forests and national parks. Outreach efforts must therefore be aimed at modifying activities, particularly land conversion for urbanization and residential development on private lands. Investigation of the design and extent of a Sierra-wide oak reserve network, similar to the LS/OG reserve network discussed above, should be implemented. Even more importantly, however, oak woodland preservation and safeguarding the habitat needs of oak-dependent species must be built into the growth plans of all Sierra foothill communities and counties.
Objective 6. Improve oak recruitment throughout the Sierra.
Recommendation 6-1. Study and revise fire and other land management practices to encourage oak regeneration.
Numerous factors, including fire management, livestock grazing, and invasion by exotic grasses have likely played roles in dampening oak recruitment throughout the Sierra. Research should focus on land management practices that might improve recruitment. Appropriate fire management is a very promising tool, and should be implemented where feasible.
SIERRA NEVADA REGION-WIDE RECOMMENDATIONS
Objective 7. Continue and expand current Sierra-wide bird monitoring efforts.
Recommendation 7-1. Recruit committed observers to continue surveying the Sierra BBS routes, including the 20 new routes generated since 1997.
BBS data provide an enormously valuable record of changes in bird abundance and distribution over time. The recent addition of more routes promises a dramatic improvement in our ability to detect changes in Sierra avian community dynamics over the coming years. The realization of this improvement, however, requires that committed observers be recruited to continue to conduct the surveys indefinitely into the future.
Recommendation 7-2. Design and implement a long-term, off-road, habitat-specific avian monitoring program.
Despite its tremendous value, the BBS protocol has substantial limitations, including its restriction to roadside survey points, and its inability to distinguish differences in avian community composition by habitat, let alone management regime. An off-road, habitat-specific monitoring program throughout the Sierra Nevada would be enormously valuable in providing baseline data on the status of many Sierra bird populations not adequately sampled by the BBS.
Recommendation 7-3. Deploy additional MAPS stations throughout the Sierra to better understand the primary demographic parameters responsible for Sierra-wide population trends of numerous species.
An additional limitation of the BBS protocol is that it provides information only on secondary population parameters, such as population size and density, rather than primary demographic parameters like productivity, fecundity, survivorship and dispersal. Primary parameters may be more useful than secondary parameters in determining the causes of population changes, and suggesting possible actions to reverse them (DeSante and George 1994). Additionally, studying primary parameters over the short-term can elucidate long-term population trajectories (Hutto 1988, Temple and Wiens 1989).
The MAPS protocol allows the estimation or indexing of primary demographic parameters, including productivity and survivorship. The 15 MAPS stations currently in operation in the Sierra will shed much light on population changes of Sierra birds, and more importantly, proximate causes of those changes (i.e., changes in productivity, indicating problems on the Sierra breeding grounds, or changes in survivorship, which could reflect problems on far-away wintering grounds). However additional stations throughout the Sierra would be invaluable in increasing the precision of parameter estimates, and in providing truly Sierra-wide data. Existing stations are concentrated in Yosemite National Park and Tahoe National Forest, leaving the southern Sierra (as well as large regions of the central and northern Sierra) completely unrepresented.
Recommendation 7-4. Implement effective monitoring efforts for habitats, species, and seasons for which current efforts are insufficient.
Population size estimates, population trends, and detailed distributional information for chaparral-inhabiting species in the foothills of the Sierra are currently unavailable, despite the fact that a substantial portion of the entire breeding range of several of these species occurs in the Sierra foothills. Examples include California Rufous-crowned Sparrow (A. r. ruficeps), California Black-chinned Sparrow (S. a. cana), California Sage Sparrow (A. b. belli), and Lawrence’s Goldfinch. Likewise, population size estimates, population trends, and detailed distributional information are also generally lacking in the Sierra for nocturnal species, particularly Long-eared Owl and most small owls. In addition, winter distributional information and the relationship between winter distribution and acorn, conifer nut and seed, and berry food crops is scanty and fragmentary. Effective long-term monitoring efforts to provide baseline data on these habitats, species, and processes should be implemented.
Objective 8. Focus hypothesis-driven research on the effects of specific land management practices on breeding, dispersing, migrating, and over-wintering birds, and on the relationships between spatial patterns of productivity, survivorship, and population trends, for selected target species.
Recommendation 8-1. Deploy additional MAPS stations in locations that will test the effects of specific land management practices on avian productivity and survival and the relationships between productivity, survivorship, and population trends.
If new MAPS stations are thoughtfully sited using spatially explicit,-landscape level habitat data, they will be enormously useful in determining the effects of specific habitat and management characteristics on avian community composition and demographic parameters. Forest composition and structure, timber harvest regimes and grazing practices are all examples of variables whose effects on avian productivity and survivorship could be elucidated with appropriately placed MAPS stations.
A second fruitful approach is to site MAPS stations in habitat types, management regimes, and geographic areas in the Sierra where population trends for a given target species are decreasing, and in analogous habitats, management regimes, or geographic areas where population trends for the same species are stable or increasing. This will allow identification of the primary demographic cause of population declines (low productivity, low recruitment, or low survivorship). When coupled in a GIS with spatially explicit habitat and weather information, data from such MAPS stations will allow strong, testable hypotheses to be formulated regarding the ultimate environmental cause of population declines, and will aid in the identification of specific management actions and conservation strategies to reverse the declines.
Recommendation 8-2. Deploy nest-monitoring studies throughout the Sierra to provide a mechanistic understanding of how various habitat variables and land management practices affect nesting productivity.
Very little is known about the effects of different land management practices, including management regimes that govern fire control, timber extraction, and livestock grazing, on the nesting success of birds in general (Martin 1992), let alone Sierra birds. The BBIRD field protocol (Martin et al. 1997) provides a clear, standardized methodology for using nest monitoring to provide a mechanistic understanding of how habitats or management practices impact productivity. At least two large-scale, multi-species nest monitoring studies are currently in progress in the Sierra, but additional studies addressing a variety of habitat variables and land management practices should be implemented throughout the region.
Objective 9. Focus research efforts on the effects on bird populations of ongoing ecological changes in the Sierra including those caused by factors both internal and external to the Sierra.
Recommendation 9-1. Encourage the collection of data that will enable prediction of how ongoing changes in forest composition and structure brought about by management actions in the Sierra will affect avian community composition and population dynamics.
Although the overall state of forest health in the Sierra may be relatively good (Franklin and Fites-Kaufmann 1996) even subtle changes in age-class distribution, structure and composition of forest stands, and patterns of forest distribution across the landscape may have far-reaching implications for avian community composition (Hejl 1994). The ongoing conversion to white fir- dominated stands throughout much of the Sierra, as well as associated changes in forest structure, is likely to have a profound effect on the abundance and distribution of many Sierra bird species. . A far better understanding of habitat preferences and the ecological factors affecting nesting success of numerous bird species is required to predict the consequences of these changes for the Sierra avifauna. Such predictions are critical for proactively focusing monitoring and conservation efforts on the species that are most likely to require them in the coming decades, before populations become critically at-risk. If implemented thoughtfully, all the research and monitoring efforts called for in Recommendations 7-1 through 8-2 will contribute to reaching this goal.
Recommendation 9-2. Encourage the collection of data that will elucidate the likely effects on Sierra bird populations of environmental factors originating from or acting outside of the Sierra.
Three environmental factors originating from outside the Sierra are noteworthy here. First is large-scale human-caused climate change, particularly that associated with increased levels of CO2 and other greenhouse gases. Relationships between climate change and ecological and demographic parameters of Sierra birds are likely to be very complex and to involve substantial time lags. Indeed, even the various manifestations of climate change itself, involving both temperature and precipitation, are likely to be complex and to show substantial seasonal variation. Precipitation, for example, could increase during winter leading to heavier snow packs and wetter conditions, but decrease (on the east side of the Sierra at least) during summer leading to drier conditions. Temperatures could be increased in some seasons and decreased in others. Initial analyses of Sierra MAPS data suggest an inverse relationship between winter precipitation and breeding productivity for several species, including certain meadow-dependent species such as Lincoln’s Sparrow and MacGillivray’s Warbler (P. Nott, pers comm.). These data suggest that increasingly dry winter conditions could lead in the short term to higher population sizes through increased productivity, but such conditions may also lead in the long term to increased drying out of meadows and subsequent populations declines through lowered recruitment due to loss of meadow habitat. The collection of long-term data relating both local and large-scale weather conditions to demographic parameters of Sierra birds is crucial for predicting likely consequences of climate change on Sierra birds. Even more importantly, demonstration of the probable ecological effects of climate change before they happen may provide one of the best means of triggering effective actions to reduce the anthropomorphic causes of climate change.
A second major environmental stressor from outside the Sierra that can effect the ecology of Sierran birds is large-scale, pervasive, airborne pollution. Detrimental effects on amphibians and piscivorous birds from acid deposition in lakes and rivers through rain and snow provide one of the best-known examples of the effects of this type of pollution. The ecological effects of smog, which has become pervasive over the southern (at least) Sierra in recent years, are virtually unknown. A third potential stressor is provided by the increased use of agricultural pesticides in the Central Valley which could negatively affect those flying insects in the Central Valley that are subsequently wind-drifted to higher elevations in the Sierra and that may provide important food sources for swifts, nighthawks, Olive-sided Flycatchers, and even Gray-crowned Rosy Finches which feed extensively on wind-drifted insects that are precipitated onto alpine snowpack. Additional research on the effects of airborne pollution and the effects of agricultural pesticides on wind-drifted insects is needed before either of these potential risks to Sierra birds can be dismissed.
A fourth factor operating outside the Sierra that could be very important in effecting Sierra bird populations is destruction and degradation of wintering habitat. Analyses presented earlier in this report suggest that Neotropical-wintering species are generally faring as well as or better than permanent resident and short-distance migrants, although declining trends in several species, including Olive-sided Flycatcher and Swainson’s Thrush, may well be caused by conditions on their tropical wintering grounds. The same data, however, suggest that a major cause of the declines in short-distance migrants may well involve adverse wintering-ground conditions in southern California, Arizona, and northern Mexico. The conclusive demonstration of such links is critical for efforts to provide habitat protection of the wintering grounds. The encouragement of research and monitoring efforts to establish such links, including efforts to use genetic evidence and trace element analysis to establish concordance between breeding and wintering areas for discreet Sierra populations should be an integral part of any effective Sierra Nevada avian conservation plan.
Objective 10. Maintain and restore habitat diversity throughout the Sierra Nevada.
Recommendation 10-1. Revise fire management regimes to mimic natural fire systems wherever possible.
Fire is critical for maintaining and restoring forest diversity. Using prescribed burns to clear out unnaturally high downed fuel loads, and then permitting low-intensity surface fires to run their course (where human structures are not threatened) can be a valuable tool for stimulating oak regeneration (see Recommendation 6-1), promoting the development of LS/OG forest characteristics (see Recommendation 3-2), and generally preserving forest heterogeneity, which is critical for maintaining the Sierra’s full complement of avian diversity.
Recommendation 10-2. Integrate components of this avian conservation plan into management plans developed by federal agencies for their Sierra landholdings and into plans being created by counties and communities in the Sierra to guide growth and residential and commercial development.
The rapid growth of the human population in the Sierra, which involved a doubling in the twenty years between 1970 and 1990, is expected to accelerate over the next few decades (Duane 1996). This will not only involve greatly increased residential and commercial development of the lower elevations of the Sierra, but vastly increased pressure on all elevations for recreational use and water resources. This growth will place further demands on the public as well as private lands of the Sierra. It is imperative that this growth be planned and regulated in ways that preserve the ecological integrity and aesthetic values of the entire range. Comprehensive range-wide planning is already underway on lands managed by the USDA Forest Service and these plans are being integrated with planning processes for the individual national parks. It is crucial that the plans being developed by federal agencies take into account the human population growth that is inevitable, but is equally critical that important management concepts developed in public plans be included in the planning process for private lands. Moreover, it is essential that the underlying concepts developed in this avian conservation plan filter into subsequent plans for both private and public lands.
The challenge before us in the Sierra, to do it right this time, has never been more critical because we may never get another chance. Yet, the information and tools we have in our hands have never been more powerful and the willingness to cooperate has never been higher. We have an unprecedented opportunity to fashion the direction that management and development in the Sierra will take over the next few decades such that the Sierra can continue to remain, for all the world to see, "The Range of Light."
We thank J. Verner, J. Robinson, J. Steele, R. Stafani, K. Purcell, and G. Studinksi for sharing their ideas about Sierra bird conservation, the personnel of the BBS for making available data and trend-estimating software, D. O’Grady and P. Nott for help with data analysis, and the numerous BBS observers and MAPS interns and contributors for their efforts in the field. We also thank the David and Lucille Packard Foundation, the National Fish and Wildlife Foundation, and California Partners in Flight for funding this project, and the Point Reyes Bird Observatory for logistical support.
This is contribution number 111 of The Institute for Bird Populations.
7. LITERATURE CITED
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Airola, D. A. 1986. Brown-headed cowbird parasitism and habitat disturbance in the Sierra Nevada. Journal of Wildlife Management 50:571-575.
Allen, B. H. 1987. Forest and meadow ecosystems in California. Rangelands 9:125-128.
Anderson, M. K. 1993. The mountains smell like fire. Fremontia 21:15-20.
Anderson, M. K. and Moratto, M. J. 1996. Native American land-use practices and ecological impacts. Sierra Nevada Ecosystem Project, Final Report to Congress, vol. II, Assessments and Scientific Basis for Management Options. University of California, Davis.
Andrews, R. S., ed. 1994. Ecological support team workshop proceedings for the California Spotted Owl Environmental Impact Statement, August 1993. San Francisco: U.S. Forest Service, Pacific Southwest Region.
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