California Partners in Flight—Oak Woodland Bird Conservation Plan
Blue-gray gnatcatcher
    Polioptila caerulea


 Section 1: Species account outline.

SPECIES: Blue-gray Gnatcatcher, Polioptila caerulea

SUBSPECIES STATUS: P. c. amoenissima

This subspecies breeds in the western United States and NW Mexico, and winters from southern California (casually from Marin County), western and central Arizona, and western Texas south to latitude 28 degrees N. in Mexico (AOU checklist 1957).

MANAGEMENT STATUS: No special status. Protected under the migratory Bird Act.

Considered a neotropical migrant landbird of special concern in California (Laymon 1995).

range map

I.  Historical references: According to Grinnell and Miller (1944), bred in California in the foothills and low mountains surrounding Sacramento and San Joaquin River Valleys, locally in river bottoms of the valleys, in the interior coast ranges, the mountains and coastal plain of southern California south to Mexican boundary, and desert mountain ranges from Providence Mountains north to White Mountains.

II.  Current breeding distribution: Attempt to obtain the most current information.

    1. UTM or lat long coordinates of sites known to contain breeding populations:
1. Type of method used in determining breeding status (by site and year).
    a.  Expert opinion:
· Believed to have been greatly reduced in lowlands due to Brown-Headed Cowbird parasitism, and to no longer breed in the Central Valley (Gaines 1974). Grinnell and Miller (1944) said BGGNs bred locally in riparian habitat in Central Valley riverbottoms, but Gaines (1974) did not find them in the Sacramento Valley during his 1973 breeding census, and believes they are no longer there. Speculates that this is due to cowbird parasitism. No observations of breeding BGGNs in the San Joaquin Valley or the Sacramento River Valley during extensive point count censuses of riparian habitat along these rivers and their major tributaries in 1998 (PRBO data).

· Extirpated from the coastal lowlands of San Diego County (Garrett and Dunn 1981, Small 1994, San Diego BBA). However this may be in the process of reversal, most likely due to cowbird trapping in efforts to save the Least Bell’s Vireo (Phillip Unitt, pers. Communication). See San Diego Breeding Bird Atlas below.

· Mill Creek, Lassen National Forest, Tehama County. Birds were not detected on point count (PRBO data 1997) or ever observed in riparian habitat, but were observed during breeding season in adjacent oak savannah and oak scrub (Anne King, pers. communication, 1997-1998).

b. Point count (singing individual encountered on 2 or more different days of census-at least one week apart):
  · Inyo County, east side of the Sierras (PRBO data 1998). · Marin County (PRBO data 1996-1998). · Mendocino National Forest, Mendocino County (PRBO data 1996-1997).

· San Luis Obispo County, central coastal region (Mike Lynes, pers. comm.).

· San Mateo County, Santa Cruz Mountains (Sisk et al. 1997).

· Tehama County, detected along Dye Creek in Sierran foothills; plots covered riparian habitat and adjacent oak savannah (PRBO data 1998).

· Yuba County, foothills of northern Sierra Nevada range (Aigner et al. 1998).

    c.  Mist netting (female with brood patch, female with eggs in oviduct, juvenile with no skull ossification before 1 August): no information.

    d.  Nest searching:

· Inyo County, eastern Sierras: common breeder in shrub-steppe habitat; during study of riparian drainages in eastern Sierras, 3 of BGGN 4 nests found were in adjacent shrub-steppe, 1 of 4 nests was in riparian habitat.
    e.  Spot mapping:
      · Camp Roberts, central coastal region, San Luis Obispo County (Tietje and Vreeland 1997).
    f.  Area search: No information

    g.  Breeding Bird Atlas:

· Marin BBA - breeds only on relatively dry ridges in the interior (Shuford 1993).

· San Diego BBA - breeds sparingly in San Diego County chaparral above 2000 feet, and occasionally in desert riparian. Recent observations in Spring Valley and in Miramar Naval Air Station suggests the species may be beginning to reoccupy this lowland habitat in response to lessening cowbird parasitism in the area (San Diego BBA). Throughout the county, far more BGGNs are currently seen during the breeding season than 20 years ago, and it is likely that the Least Bell’s Vireo cowbird trapping program is responsible for this change (Phillip Unitt, pers. communication).

· Monterey BBA - Local declines have occurred in Monterey County. Were considered abundant in early 1900’s in upper Salinas Valley (Willet 1908 in Tenney 1993), and the numbers now are very low and local. Declines believed to be due to habitat loss and cowbird parasitism. Tenney (1993) recommended a cowbird control project. Fairly common breeder in chaparral and arid woodlands throughout the Santa Lucia Mountains, Sierra de Salinas, and northern Galibar. Also partial to upland foothills covered with small oaks and open chaparral. Few reside east of Salinas River, which is mostly grassland and pine-oak woodlands. Seldom nest in coastal scrub of coast (Tenney 1993).

· Sonoma BBA – Breed in eastern portion of county, and are for the most part absent from the central and western parts (Wigh 1995).

    h.  BBS route:
· most birds per route in west are from inner coast ranges and Sierran foothills of California (S. Droege pers. comm. in Ellison 1992).
    i.  Other/Local opinion:
· Small (1994) states that this species has been eliminated from former breeding range in lowlands of Orange County. However, Hamilton and Willick (1996) make no mention of this, saying only that BGGNs nest locally in mixed oak/chaparral habitat in mountains, foothills, and in coastal chaparral.

· Local, and uncommon to fairly common breeder in Santa Barbara County. Most numerous in hills bordering the Upper Santa Ynez River, and along the crest of the Santa Ynez Mountains (Lehman 1994).

· Very rare breeder in California’s Deep Canyon (between Colorado Desert and Santa Rosa Mountains); only a few pairs nesting in the pinyon-juniper and chaparral (Weathers 1983).

· Said to be found year-round in the following National Forests: Angeles, Cleveland, Los Padres and San Bernadino; summers (breeds?) in El Dorado, Inyo, Klamath, Lassen, Mendocino, Modoc, Sequoia, Shasta-Trinity, Sierra, Six Rivers, Stanislaus, Tahoe and Toiyabe National Forests (Timossie 1990, in USDA Forest Service 1994).


I.  Average territory size: At Hastings Reserve, located in Central Coast Ranges of California, Monterey County, territories averaged 4.5 (2.2-7.4) acres or 1.8 (0.9-3.0) hectares, n=9 (Root 1969).

II. Time of occurrence and seasonal movements: Northerly populations show long distance migration (Ellison 1992).

III.  Migration stop-over needs/characteristics: IV.  Nest type: Cup with relatively high walls, outside of which consists of lichens and spider webs or caterpillar silk, the lichen which camouflages it in most circumstances (Weston 1949). Interior consists of fibrous materials such as forb and grass stems, bark strips, becoming progressively finer towards interior of the nest, where materials include willow catkins, plant down, paper, cocoons, hair, feathers (Weston 1949, Root 1969).

V.  Foraging strategy: Forage throughout height of the vegetation but concentrate in the foliage zone; in chaparral they tend to use the subcanopy zone more. (Root 1967). Hop rhythmically from perch to perch, capture prey by gleaning, lunging, hovering, and hawking (Shuford 1993). They subdue large prey by beating them against branches. In CA feed more in evergreen foliage in spring than later in summer. When deciduous oaks are barren or in the process of developing new foliage, i.e. during early spring, live oaks were used as foraging substrates more frequently than at other times (Root 1967).

VI.  Displays: To other males, male assumes tail-spread posture, tail is fanned out exposing white outer rectrices; tail held horizontally and wagged from side to side; sometimes does short undulating flights w/ spread tail displayed; during this sings sometimes but more often gives it’s "peeew" call. During most intense disputes, one male flies directly at the other, where often they meet each other in midair and ascend w/ breasts nearly touching in vertical flight (Root, 1969).

VII.  Social Organization:

· 12 pairs on a 56.1 acre (22.7 hectare) plot at Hastings Reserve in California (Root 1969).

· As determined by spotmapping at Camp Roberts, San Luis Obispo County, from 1994-1995, 4.4 territories per 100 acres (Tietje and Vreeland 1997).

· Home range of 4 hectares found in Florida (Fehon 1955 in Ellison 1992). Area patrolled and defended earlier in nesting season larger than area defended later in season (Root 1969). Discrepancies have been found in territory size, this might reflect spacing (Ellison 1992).

· In pinyon-juniper habitat in NM, found in relatively low densities, averaging less than 2 pairs per 35 hectares (Goguen 1996).

· Highest densities overall were found in southeastern U.S. (Ellison 1992).

· Highest densities in west were recorded on a BBC plot in blue oak woodland in central California with 71 territorial males/km squared and in pinyon-juniper woodland in Utah (Williams 1979 in Ellison 1992). Territories which contain large areas of open woodland tend to be larger, suggesting that territory size is related to the amount of tree foliage present (Root 1967).

· Lowest densities on Breeding Bird Census plots were in disturbed upland habitats such as clear cuts, reclaimed strip mines, and abandoned agricultural land (Ellison 1992).

B.  Mating system: Monogamous.
    C.  Delayed breeding (where are SY birds?): no information.

    D.  Post fledging biology of offspring (where do they go and when?): Adults feed young infrequently after day 16 and at times up to 4 weeks; young may stay in parents’ territory up to a month after fledging even when not being fed. During June, most fledglings led to chaparral, while during July fledglings spent more time in the live oak woodlands (Root 1967). Some immatures shift into nearby areas and/or habitat not used during breeding season. No data on initial dispersal from natal site; may wander upslope as far as the mixed conifer zone in the Sierras (Grinnell and Miller 1944).

    E.  Post breeding social behavior (mixed species flocks, or simply migrate away?): no information.

VIII.  Clutch size: At Hastings in California, mean = 4.25, n=20 (Root 1969); across multiple states, mean = 4.35, n=160 (Ellison 1992).

VIX.  Incubating sex: Both sexes incubate, roughly equal amounts of time, although only female develops brood patch. Female incubates at night (Ellison 1992)

X. Incubation period: mean 13 days, range 11-15 (Ellison 1992)

XI.  Nestling period: Across multiple states: 10-15 days (Ellison 1992); at Hastings in CA, 12-13 days(Root); in east, 10 to 12 days (Weston 1949).

XII.  Development at hatching: altricial

XIII.  Number of broods: More pairs are single brooded than double (Ellison 1992). One third of 12 pairs at Hastings, California had second broods, and pairs had multiple attempts when nests failed: 2 pairs observed making 7 attempts in one season (Root 1969). At Hastings breeding season goes from late March to late August (Root 1969), allowing multiple attempts.

XIV.  Who tends the young: Brooding done mostly by females, male occasionally covers young (Root 1969, Ellison). Both parents feed young (Ellison), male feeds more in earlier nestling phase when female does more brooding, although males appear to feed second broods more than first broods (Root 1969).
XV.  Diet: XVI.  Wintering ground needs and distribution: BREEDING HABITAT AND NEST SITE CHARACTERISTICS:

I.  Overview of breeding habitat: (e.g. oak woodland vs. oak savannah, age of stand, dominant species, plant species diversity, structural diversity/variability):

· Most favorable habitat in California is thought to be blue-oak covered hill slopes and chaparral edges mingled with oaks of several species or with diversified arroyo-edge cover. Occurs sparingly in the adjacent chaparral (Root 1967).

· Occurs in the open chaparral that normally borders stands of pinyon-juniper and/or oak woodlands (Grinnell and Miller 1944). Some territories although adjacent to woodland may contain entirely chaparral (Root 1967); however Root (1967) found that the one territory which did so was centered around a gulch where the chaparral (chamise and buckbrush) grew much larger, up to 9 feet, so the vegetation actually structurally resembled oak scrub. Breeds in blue oak woodland in interior coast ranges of CA.

· In Marin County, is attracted to slopes w/ open stands of small valley oaks w/ adjacent patches of coast live oaks and openings w/ low brush (Shuford 1993).

· In Monterey County, prefers extensive stands of oaks varying from live oak woodland, mixed live oak-deciduous oak woodland, dense oak scrub, and open stands of mature deciduous oaks (Root 1967).

· In Mendocino County, Mendocino National Forest, habitat is Brewers Oak Forest, dominated by small, scrubby Brewers Oaks.

· Less commonly occurs (and often locally) in riparian habitat, i.e. in willow thickets, sycamores and cottonwoods, and then usually where there is adjacent chaparral (Grinnell and Miller 1944). Root found the one pair which occured in riparian woodland at Hastings made frequent foraging trips into the chaparral and oak woodland nearby.

· In desert mountains, pinyon, juniper and Purshia shrubs provide comparable conditions (Grinnell and Miller). BGGN is a common species in pinyon-juniper habitats in these desert ranges (Garrett 1981).

· Also breeds in California’s shrub-steppe habitat (i.e. east side of the Sierras, Inyo County). Occasionally nests in the riparian strips that cut through the shrub-steppe. When nesting in shrub-steppe that has adjacent riparian, may utilize this wetter, perhaps more diverse (for prey species?) riparian habitat for foraging (Sacha Heath pers. comm., PRBO data).

· In the Sierras, breeds in blue oak savannah, digger pine-oak, chaparral edges mingled with oak, and in riparian deciduous forests if adjacent to oak woodland or chaparral (Verner and Boss, 1980 in USDA Forest Service 1994).

· Mendocino National Forest: Brewer’s Oak forest. Brewers Oak, the dominant tree and shrub, is a small, winter-deciduous scrubby oak (Tom Gardali, pers. comm., PRBO data).

· Broad-sclerophyll "canyon" forest of California, which structurally resembles the flood plain forests which this species prefers in the east, is only marginal habitat in CA (Root 1967).

· Generally rare or absent from habitats dominated by needle-leaved conifers. Also absent from live oak woodlands in the fog belt along the Pacific Coast, just 14 miles west of Hastings Reservation where they breed (Root 1967).

· Have been found to shift their habitat use in response to arthropod prey availability (Root 1967) - when arrive in Monterey County in March, April, they concentrate foraging efforts in the evergreen foliage of live oaks and chaparral. By late April, when deciduous oak foliage is well developed, they shift most foraging to these woodlands through July. Fledglings led to dense evergreen foliage partly for protection from predators, and later wander to nonbreeding habitats. By August adults and juveniles leave deciduous oak woodlands for adjacent live oak woodlands and chaparral.

· Breeds in very different habitats elsewhere in US, particularly in east (Ellison 1992).

    A.  Substrate (species):
      · Nest in a variety of substrates, suggesting that plant species may not be a critical factor in nest site selection (Root 1967, Ellison 1992).

      · In Monterey (Hastings Reservation), 90% of nests were in deciduous oaks, n=64 (Root 1967). Interestingly, 2/3 nests that were in live oaks had been partially defoliated by tent caterpillars and superficially resembled deciduous oaks.

      · Also use chamise shrubs at Hastings, in 1 case even when several deciduous oaks occured in their territory (Root 1967).

      · One pair with territory centered in chaparral built 2 nests in live oaks, 1 in a buck brush shrub, and one in deciduous oak (Root 1967).

      · In Inyo County, where research and monitoring was being conducted primarily in riparian habitat immediately adjacent to shrub-steppe, 3 of 4 nests found were in Artemisia tridentata (Big Sagebrush), and one was in a Black Cottonwood in the riparian habitat (PRBO data).

      · In foothills of Sierra Nevada (Tuolumne County), BGGNs have been reported to build nests in pine and alder trees (Chamerlin 1901 in Root 1967).

      · Even reported using eucalyptus (Weston 1949).

    B.  Height of nest:
· Variability suggests that height may also not be a critical factor in selection of nest sites. Height at which nests were placed above ground is related to the height of the nest tree, with placement towards the top of the shrub or tree. Height of 66 nests at Hastings Reservation varied from 3to 34 feet, w/ 79% between 7 and 23 feet high. Nests in chaparral were near top of the shrub, while oak nests were at least 3 feet below the top, and usually at least a third of the way up the tree (Root 1967).

· In non oak-woodland: Inyo County, 1 nest in riparian was 10 meters high in Black Cottonwood, 3 nests shrub-steppe in Big Sagebrush were just over 1 meter high (upper third of shrub) (PRBO data).

C. Height of plant: No data for oak-woodland. However in shrub steppe/riparian habitats in Inyo County, 3 ARTRs were b/w 1 and 2 meters high; black cottownwood 17 meters high (PRBO data).   D. Nest concealment: Appears to not be based on actual foliage concealment, but based on camouflage, as lichen covered nests might be completely exposed but easily unnoticed (Root 1967, Ellison 1992). Lichen is standard, but in burned areas, pieces of scorched bark are used that may serve the same purpose (Chamberlin 1901 in Shuford 1993). In Inyo County, the 4 nests were poorly concealed in terms of percent foliage cover, supporting above idea (PRBO data 1998).
II. Vegetation surrounding the nest (Importance of each category may differ by species)
    A.  Canopy cover:
      · At Hastings Reservation, deciduous oaks occur in open stands, with more extensive areas dominated by oak scrub (8 to 24 feet tall) (Root 1967).

      · At Mendocino National Forest, tree cover (determined by vegetation assessment taken at point count stations where BGGNs occur) EXPAND

      · In shrub-steppe, no canopy cover. In adjacent riparian, high canopy cover (74-100 percent).

      · However, in northeastern US they prefer closed canopy forests (Ellison 1992). May have specific needs in upland eastern US and midwestern US forests, such as canopy openings, but more study is needed (Ambuel and Temple 1983 in Ellison 1992, Robbins et al. 1989).

    B.  Dominant plant species in canopy:
      · At Hastings Reservation (Root 1967): Quercus Douglasii (deciduous oak) with some Quercus lobata, some coast live oaks (Quercus agrifolia). Territorial boundaries extended throughout breeding season to include the dense clumps of evergreen vegetation to which fledglings are led by the adults; these areas may provide the fledglings better protection from predators than the more open deciduous oaks and chaparral (Root 1967).

      · At Mendocino: Brewers Oak, followed by Oregon White Oak, with some Black Oak and Ponderosa Pine (PRBO data).

    C.  Average shrub cover:
      · Shrubs cover considered an important aspect (see Breeding Habitat Overview above).

      · In a point count study in the Santa Cruz Mountains of central coastal California, abundance not significantly different between oak patches surrounded by chaparral and oak patches surrounded by grassland (Sisk et al. 1997).

      · In Inyo County, east side of Sierras, shrub cover averaged 63% (n=4, 3 nests in shrub-steppe, 1 nest in adjacent riparian).

    D.  Dominant shrub species:
      · At Hastings, chaparral is dominated by chamise (Adenostoma fasciculatum) on dry slopes, and buckbrush (Ceanothus cuneatus) where it is wetter (Root 1967). ·· In eastern Sierras, shrubs are predominately Big Sagebrush (Artemisia tridentata), Rabbitbrush, Coffeeberry (PRBO data).

      · In desert ranges, dominant shrub is Antelope Brush (Small 1994).

      · At Mendocino National Forest, dominant species were Brewers Oak and manzanita (PRBO data).

    E.  Average forb cover: Inyo County, eastern Sierras, cover <10%, n=4 (PRBO data). No other information.

    F.  Dominant forb species: No information.

    G.  Ground cover: No information.

    H.  Slope: Inyo: 2-10%. In Maryland a significant negative correlation was found between BGGN abundance and slope (Robbins et al. 1989).
    I.  Aspect: Inyo County, eastern Sierras, 18-90 degrees (n=4) (PRBO data).

    J.  Tree DBH:

      · In pinyon-juniper habitat in New Mexoci, mean dbh=17.4cm, n=92 (Bbird data).

      · In Arizona, mean dbh=22.8cm, n=67 (Bbird data).

      · In Minnesota, mean dbh=46.7 (Bbird data).

      · In Ohio, mean dbh=38.4, n=3 (Bbird data).

    K.  Snags:
      · Only data comes from Inyo County, eastern Sierras, n=4. Mean percent snags = 0%; mean percent logs/stumps=12.5% (PRBO data).

      · In Maryland a significant negative correlation was found between BGGN abundance and number of standing dead trees (Robbins et al. 1989).

    L.  Distance to water: Ellison (1992) reports no published information.
III.  Landscape factors:
    A.  Elevation:
      · Prefer foothills and low mountains (Grinnell and Miller 1944).

      · In western Sierra Nevada, said to occur below 4000 feet (approx. 1300m); in eastern Sierra Nevada below 2300 feet (approx. 800m) (Zeiner et al. 1998 in USDA Forest Service 1994); other literature stated that they breed up to 6000 feet (approx. 2000m) in oak woodlands (Small, 1994).

      · In coast range breeds up to about 1500m (Garrett and Dunn 1981).

      · In desert slopes of mountains and in desert ranges, occurs up to 7000 feet (Small 1994).

    B.  Fragmentation: Has been suggested that BGGNs affinity to edge habitats may be partially responsible for large amounts of nest predation by jays and for the high parasitism rates (USDA Forest Service 1994). However, a study in oak woodland in San Mateo County, CA revealed that abundance of BGGNs from point count data was neither positively or negatively associated with amount of edge (Sisk et al. 1997).

    C.  Patch size: No detailed information.

    D.  Disturbance (natural or managed): (e.g. floods, fires, logging):

      · The clearing of woodland (eliminating substrates) would probably reduce BGGN populations; fires might open up habitat to their liking (Shuford 1993).

      · In a 2 year post-thinning study of blue oak woodlands in the foothills of the Northern Sierra Nevada range, BGGN abundance was not shown to be positively or negatively affected by thinning for firewood harvesting; however relative abundances overall were low and this might account for the lack of relationship (Aigner 1998).

    E.  Adjacent land use: Frequently adjacent to grazing, as California’s oak woodlands are predominately privately owned and grazed (Aigner 1998).
V. Other:

SPECIAL FACTORS: Factors influencing a species occurrence and viability.

I.  Brood parasitisim:

    · One of the smallest regular hosts of the BHCO (Weston 1949).

    · As early as the 1910’s and 1920’s, this species was observed being frequently parasitized, with 10 out of 12 nests in Mississippi parasitized (Weston 1949).

    · At Hastings Reserve, Monterey County, 6 of 22 nests in 1963 consisted of only 1 cowbird young and no host young; this is only 4 years after BHCOs were first observed at the Hastings Reserve (Root 1967).

    · 2 of 4 nests in Inyo County, Eastern Sierras (shrub-steppe/riparian) were parasitized in 1998 (PRBO data).

    · In pinyon-juniper woodland in New Mexico, where cowbirds and gnatcatchers have existed sympatrically for a long time, 76% of 83 gnatcatcher nests found were parasitized. Parasitism was responsible for the failure of 58% (48 of 83) of the nests. Two nests that were parasitized w/ one egg before laying buried the cowbird egg into their nest bottom but did not desert. Of nests parasitized during egg-laying, 45% were deserted. None of the 25 nests that accepted cowbird eggs during laying fledged host young; 16 successfully fledged a cowbird. All cowbird eggs laid after clutch completion were accepted. Some which accepted cowbird eggs during incubation fledged their own young. Parasitism rates were lower late in the breeding season, and nesting success of unparasitized nests was higher (Goguen and Matthews 1996).

    · Goguen and Matthews (1996) suggested desertion as the strategy used by BGGNs in response to cowbirds, as pairs that deserted were sometimes able to raise cowbird-free clutches. Gnatcatchers are very common hosts and typically raise none of their own young when parasitized, yet no other anti-parasite behaviors have been reported. Of 48 parasitized pairs, 19 pairs deserted at least one nest; however, only 7 deserters were relocated by nest searchers. 6 of the 7 deserters experienced at least one subsequent unparasitized nest, although it often took 2 or 3 more attempts before this was achieved. Nest desertion is often more common response of smaller hosts that perhaps cannot physically eject the eggs. Also, BGGNs show nest moving - break down of old nest as primary material source of next attempt.(Weston 1949). However, effect of late nesting due to desertion parasitized nests on adult survival or on fledgling survival is unknown (Goguen and Matthews 1996). Also, desertion is not always the response to parasitism; Goguen and Matthews (1996) suggest that they may only desert when the parents observe the nest being disturbed by a cowbird. Also suggest that this species may be in an evolutionary transitional stage..

    · Parasitism rates and cowbird abundance did not differ b/w grazed plots and plots ungrazed for 20 years, but the ungrazed plots had nearby feedlots/grazing which were ideal for BHCOs (Goguen and Matthews 1998). Reflects need for landscape approach to deal with indirect affects of grazing (i.e. cowbirds) on songbird populations.

    · Parents fed BHCO nestlings (which often meant just one BHCO in a nest) at rate similar to that of a normal brood. Two individual cowbird fledglings (separate pairs/nests) each averaged 28.0g in body weight, while the total weight of 5 BGGN fledglings (max normal brood size) was 29.3g (Root 1967)

    · Pressure this high cannot be applied to the population as a whole without causing it’s demise (Goguen and Matthews 1996). Perhaps is only being applied to restricted areas (lowlands adjacent to grazing)?

II.  Dietary: Purely insectivorous. See pesticide use below.

III.  Sensitivity to human-induced disturbance: Said to desert nests easily (Root 1969).

IV.  Pesticide use: Many of its prey items are agricultural pests (Terres 1980 in USDA Forest Service 1994), so would be affected by pesticides. No reported studies/information on this subject.

V.  Predators:

    · Probable predation by Western Scrub-jay was said to be the major factor responsible for nest loss at Hastings Reserve (Root 1969).

    · In 1961 after a population outbreak of tent caterpillars that defoliated the oaks, all located BGGN nests failed, and many individuals did not seem to even be engaged in nesting activity, however nest searching was not as intense as in 1963 (Root 1969). ·· Documented nest predators in west include WESJ, YBMA (Ellison 1992).

    · Inferred predators include American Crow, raccoons, squirrels, rat snakes. Also probable are snakes and other small climbing mammals such as Peromyscus mice (Ellison 1992), and possible predator on adults are Loggerhead Shrikes (personal observation).

    · Adults mob potential predators.

    · Predation appears important in regulating nest success but apparently has little effect on adult populations (Ellison 1992).

VI.  Exotic species invasion/encroachment: abundance of BGGNs in Marin as determined by point count censuses does not appear to be negatively affected by presence of Scotchbroom (PRBO data).

VII,  Other: Temperature and potentially vegetation appear to be the major factors limiting the winter range (Root 1988 in Ellison 1992). Gnatcatchers wintering in temperate an subtropical areas may be sensitive to severe winter weather (Weston 1949), as has been found for other temperate winterers the same size (Laurenzi et al. 1982). A two-week freezing spell in Florida in 1940 caused the complete disappearance of BGGNs that winter, and none were seen until spring migrants returned (Weston 1949).


· BBS Data: nonsignificant positive trend over entire state. However, this trend comes with warning on the website, that results for low relative abundance species such as BGGN must be viewed with caution, and especially may be less accurate over long-term. Coupled with the fact that the trend is not statistically significant, this trend may not be meaningful.

· Said to have recently expanded range into northeastern CA in Lassen County, but source of this uncertain (USDA Forest Service 1994).

· Nesting range expansion in North/Northeast US (Ellison 1992, Ellison 1993).

· Recent pop trends in US (1966-1987) include increases in E and W regions and a decline in the Central region, none judged significant (S. Droeg pers. comm in Ellison 1992).


I.  Age and sex ratios: no information.

II.  Productivity measure(s):

    · 24% of nests fledged BGGN young at Hastings Reserve, CA (Root 1969).

    · Of 12 females studied, 58% fledged young from 1st nest attempt, and 33% fledged young from 2nd broods (Root 1969 in Ellison 1992).

    · Over 64% of all nests found in Vermont in 1988 produced fledglings, but only 42% found mid-incubation or earlier fledged young. Note: Mayfield method (1961) not used to determine nest success at any of above examples.

    · In Eastern Sierras, Inyo County, 1 of 4 nests believed to have fledged, 1 abandoned during building, 1 abandoned with 3 out of the 4 eggs being BHCO eggs, and 1 nest depredated with 2 of 4 eggs being cowbird eggs (PRBO data).

    · Productivity in NM: 7 unparasitized pairs averaged 3.0 young fledged (all successful); 16 pairs that accepted cowbirds during laying on averaged fledged 0.00 BGGN and 1.00 BHCO young; 5 pairs that accepted cowbird eggs laid during incubation period averaged 1.60 BGGN and 0.40 BHCO young fledged; 4 pairs that deserted and moved nests in response to parasitism fledged 2.25 BGGN and 0.25 BHCO young on average.

III.  Survivorship: Oldest recaptured bird was 4yr. 2 mo. old when last encountered (Klimkiewicz et al. 1983 in Ellison 1992).

IV.  Dispersal: no data.


ASSOCIATED SPECIES: A list of other species that would benefit from management of the target species.

· In oak woodland, associated species may include: California Quail, Ash-throated Flycatcher, Red-shafted Flicker, Western Scrub-jay, Bushtit, Bewick’s Wren, Western Bluebird, Oak Titmouse, California Towhee, Spotted Towhee, and Hutton’s Vireo, among others (Sisk et al. 1997).

· In pinyon-juniper habitat in New Mexico, associated species included Western Wood-pewee, Solitary Vireo, Western Tanager, Bushtit, Spotted Towhee, Plain Titmouse, and Western Scrub-jay (Goguen and Matthews 1998).

MONITORING METHODS AND RESEARCH NEEDS: Recommend methods that will address immediate needs as well as those most appropriate to monitor how effective the proposed management recommendations will be.

· Certainly more comprehensive knowledge of statewide range is needed. Need extensive oak woodland point count surveys throughout the state, much as is being done with riparian habitat statewide.

· Need to locate specific areas for more intensive monitoring research to be conducted, specifically nest searching.

· It seems necessary to determine and monitor productivity of populations, both those which do not occur in the lowlands, and also any remaining populations in such lowlands. Found very little literature on reproductive success of BGGNs in CA, particularly any recent studies in oak woodland. Found nothing on reproductive success in populations free of BHCO parasitism.

· As it has been suggested that the parasitism rates found in BGGNs will not allow for a sustainable population, it is necessary to immediately determine the degree of parasitism of this species statewide and across habitats, to identify strong populations as well as once which are possibly declining due to cowbirds.

· If it appears that the degree of parasitism is non-sustainable, it may then be necessary to implement cowbird control in an extreme case. This is proving effective in San Diego County, where cowbirds are being controlled not to help BGGNs but to help Least Bell’s Vireo, and BGGNs are one of possibly many species gaining from this. In areas where cowbird control programs are put into affect, it is crucial to monitor the effectiveness of such a strategy.

· It is necessary to address landscape issues which are currently affecting the degree of parasitism found.

Section 2: Action plan summary. Summarize the above information into concise statements under each section.

STATUS (from subspecies, trend, local extirpations, state and federal lists, etc.)

· No federal or state special status. However is classified as a neotropical migrant landbird of special concern in California (Laymon 1995).

· BBS data reveals a nonsignificant positive trend over entire state. However, this trend comes with warning on the website, that results for species with low relative abundance, such as BGGNs, must be viewed with caution, and especially may be less accurate over long-term. Coupled with the fact that the trend is not statistically significant, this trend may not be meaningful.

· believed to have been extirpated and/or greatly reduced from the following regions: from riparian habitat in the Central Valley where they once bred locally, from lowlands of San Diego County, from Salinas Valley in Monterey County, and possibly in Orange County. These lowland extirpations are all thought to be the result of Brown-headed Cowbird parasitism, to which they are very susceptible. This species appears to be returning to the lowlands of San Diego County in response to an active cowbird control program (implemented for Least Bell’s Vireos).

HABITAT NEEDS (e.g., elevation, patch size, breeding habitat characteristics, disturbance). The following is specific to BGGNs in California, as their habitat associations and requirements are very different in the eastern portion of the country.

· Elevation: Thought to prefer foothills and low mountains according to Grinnell and Miller (1944), probably as a result of habitat. In some areas of the Sierras may breed up to 4000-6000 feet; in Sierras may breed up to around 1500 meters, or 4500 feet. May occur even higher in desert ranges.

· Most favorable habitat is blue-oak covered hill slopes and chaparral edges mingled with oaks of several species or with diversified arroyo-edge cover; also occurs sparingly in the adjacent chaparral (Root 1967). Appear to prefer a matrix of woodland (often including both live and deciduous oaks) and shrubs, sometimes including grasslands as well or instead of the chaparral. Often the oaks/trees are fairly shrubby in structure.

· Little information on patch size. Does not need extensive woodland patches and usually do not choose closed canopy forests. Prefer open matrix of trees and shrubs.

CONCERNS (e.g., productivity, brood parasitism, habitat loss, lack of information, wintering distribution, pesticide use)

· This species is highly vulnerable to parasitism. Parasitism rates very high (too high) in areas populated by cowbirds, within this state and in others as well. Much of oak woodland habitat where they breed is of a nature suitable for cattle grazing, which increases BHCO range and consequent parasitism rates.

· Lack of current information exists on productivity as determined by nest finding and monitoring; most nest searching done in previous decades or in other habitat types and may not reflect status of this species in California's oak woodlands today.

· Many prey items are known agricultural pests, which may make pesticides an issue in the diet of this purely insectivorous species, although no study has specifically been done.

· Winters in chaparral and desert scrubland in southern California. Also winters in a variety of both scrub and forest habitats in Mexico. Habitat loss in California, i.e. conversion of such habitat to agricultural or grazed lands, has the potential to affect this species' overwintering survival, as does deforestation in Mexico.

OBJECTIVES (e.g., increase distribution, identify healthy breeding populations, increase available habitat, guide restoration efforts to benefit species)

· Determine more complete range distribution across state, as well as this distribution across different habitat types.

· Identify healthy breeding populations where parasitism rates are low or nonexistent and productivity levels are sustainable, and determine what makes those sites optimal (implement monitoring studies).

· Increase amount of oak woodland habitat that is not vulnerable to development or conversion into agriculture or grazing. Increase habitat acquisition of various types of oak woodland for preservation, including areas far enough from feedlots to prevent or reduce the problem of parasitism locally.

· Implement cowbird control locally, if determined necessary, and in areas where associated species will benefit. Implement this in conjunction with land acquisition and/or protection, in areas which will not require long-term cowbird control in order for initial cowbird control to be successful.

ACTION (e.g., acquire and restore habitat, specific management and restoration recommendations, specific research and monitoring needs, specific land protection recommendations):

· Statewide census of oak woodland habitats to determine abundance and distribution of BGGNs and associated oak woodland species.

· Statewide census of other habitat types within California in which BGGNs breed (desert, shrub-steppe, pinyon-juniper).

· Set up Breeding Bird study plots to monitor reproductive success and parasitism rates in select oak woodland. Ideally, set up some plots in lowland areas that contain BHCO populations, and others in more pristine habitat.

· Habitat restoration in historically grazed areas.

· Habitat acquisition.


Aigner, P.A., W.M. Block and M.L. Morrison. 1998. Effect of firewood harvesting on birds in a California oak-pine woodland. J. Wildl. Manage. 62(2): 485-496.

Ambuel, B. and S. Temple. 1983. Area dependent changes in the bird communities and vegetation of southern Wisconsin forests. Ecology 64:1057-1068.


Chamberlin, C. 1901. Some architectural traits of the Western Gnatcatcher. Condor 3: 33-36.

Ellison, W.G. 1992. Blue-gray Gnatcatcher (Polioptila caerulea).In The Birds of North America, no. 23, 1992. (A. Poole and F. Gill, Eds.). The Academy of Natural Sciences, Philadelphia, and The American Ornithologists’ Union, Washington, D.C.

Ellison, W.G. 1993. Historical patterns of vagrancy by Blue-gray Gnatcatchers in New England. J. Field Ornithol., 64(3):358-366.

Fehon, J.H. 1955. Life-history of the Blue-gray Gnatcatcher (Polioptila caerulea caerulea). Ph.D. dissertation, Florida State University, Talahassee.

Gaines, D. 1974. A new look at the nesting riparian avifauna of the Sacramento Valley, California. Western Birds 5(3): 61-80.

Garrett, K. and J. Dunn. 1981. Birds of southern California: status and distribution. p. 290-291 Artisan Press, Los Angeles, California.

Goguen, C.B. and N.E. Matthews. 1996. Nest desertion by Blue-gray Gnatcatchers in association with Brown-headed Cowbird parasitism. Animal Behavior 52:613-619.

Goguen, C.B. and N.E. Matthews. 1998. Songbird community composition and nesting success in grazed and ungrazed pinyon-juniper woodlands. 1998. J. Wildlife Management 62(2):474-484.

Grinnell, J. and A.H. Miller. 1944. The distribution of the birds of California, pp.367-369. Pacific Coast Avifauna no. 27.

Hamilton, R.A. and D.R. Willick. 1996. The birds of Orange County: status and distribution. Sea and Sage Press, Sea and Sage Audubon Society, Irvine, CA.

Harris, S.W. 1991. Northwestern California birds. Pp 173-174. By Stanley Harris.

Hays, L.R. 1983. Disturbed coastal floodplain. American Birds 37:38-39.

Hutto, R.L. 1980. Winter habitat distribution of migratory landbirds in western Mexico with special reference to small, foliage gleaning insectivores, pp. 181-203 in Migrant birds in the Neotropics: ecology, behavior, distribution, and conservation (A. Keast and E.S. Morton, Eds.), Smithsonian Institution Press, Washington, DC.

Laurenzi, A. W., B. W. Anderson, AND R. D. Ohmart. 1982. Wintering biology of the Ruby-crowned Kinglet in the lower Colorado River Valley. Condor 84:385-398.

Laymon, S. A. 1995. Neotropical migrant landbirds of special concern in California. Kern River Research Center unpublished report to California Partners in Flight. Weldon, CA.

Lehman, P.E. 1994. The birds of Santa Barbara County, California. University of California at Santa Barbara Vertebrate Museum.

Mayfield, H. 1961. Nesting success calculated from exposure. Wilson Bulletin 73: 255-261.

McCaskie, G., P. DeBenedictus, R. Erickson, and J. Morlan. 1979. Birds of northern California: an annotated field list. Golden Gate Audubon Society, San Francisco.

Neotropical Migratory Bird Reference Book Vol. 1. USDA Forest Service, Pacific Southwest Region, Fisheries, Wildlife and Rare Plants Staff. 1994.

Nice, M.M. 1932. Observations on the nesting of the Blue-gray Gnatcatcher. Condor 34:18-22.

Phillips, A.R. The known birds of North and Middle America, part II. by Allan R. Phillps.

The AOU checklist of North American birds. Prepared by a commitee of the American Ornithologist’s Union, 5th edition, 1957, published by the AOU.

Robbins, C.S., D.K. Dawson, and B.A. Dowell. 1989. Habitat area requirements of breeding forest birds of the Middle Atlantic states. Wildlife Monographs 103:1-34.

Root, R.B. 1967. The niche exploitation pattern of the Blue-gray Gnatcatcher. Ecological Monographs 37(4):317-349.

Root, R.B. 1969. The behavior and reproductive success of the Blue-gray Gnatcatcher. The Condor 71:16-31.San Diego County breeding bird atlas.

Sauer, J.R., J.E. Hines, G. Gough, I. Thomas, and B.G. Peterjohn. 1997. The North American breeding bird survey results and analysis. Version 96.4. Patuxent

Wildlife Research Center, Laurel, MD.

Shuford, W.D. 1993. The Marin County breeding bird atlas: a distribution and natural history of coastal California birds. California Avifauna Series 1, Bushtit Books, Bolinas, California. pp. 314-316.

Sisk, T.D., N.M. Haddad, and P.R. Ehrlich. 1997. Bird assemblages in patchy woodlands: modeling effects of edge and matrix habitats. Ecological Applications 7(4): 1170-1180.

Small, A. 1994. California birds: Their status and distribution. p. 190-191 (publisher?)

Tenney, C. 1993. Atlas of the breeding birds of Monterey County, California. Pp 282-283. Roberson, R. and Tenney, C., Eds. Monterey Peninsula Audubon Society, Carmel, CA.

Terres, J. K. 1980. The Audubon Society encyclopedia of North American birds. A. Knopf, New York. 1109pp.

Tietje, W.D., and J.K. Vreeland. 1997. Vertebrates diverse and abundant in well-structured oak woodland. California Agriculture 51(6):8-14.

Timossi, I. 1990. California's statewide wildlife habitat relationships systems. California Department of Fish and Game. Computer database for the IBM personal computer. June 1992 version.

Verner, J. and A. S. Boss, Eds. 1980. California wildlife and their habitats: western Sierra Nevada. U.S. Department of Ag., Forest Service, Pacific Southwest For. and Range Exp. Station, Gen. Tech. Report. PSW-37. 439pp.

Weathers, W.W. 1983. Birds of southern California’s Deep Canyon, by the Regents of  the University of California, p. 181.

Weston, F.M. 1949. Blue-gray Gnatcatcher, pp. 345-364 in Life histories of North  American thrushes, kinglets and their allies (A.C. Bent, Ed.). U.S. National Museum Bulletin No. 196.

Wigh, A. 1995. Sonoma County breeding bird atlas. Burridge, B. Ed. Madrone Audubon

Society, Inc., Santa Rosa, CA.

Williams, P.L. 1979. Deciduous oak woodland. American Birds 33:82-83.

Zeiner, D.C., W. Laudenslayer, Jr., K. Mayer, and M. White, eds. 1990. California's wildlife, Vol. 2: Birds. California Department of Fish and Game, Sacramento, 732 pp.

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