California Partners in Flight Riparian Bird Conservation Plan
Wilson's Warbler (Wilsonia pusilla)
Prepared by: Chris Otahal (firstname.lastname@example.org)
21 Vineyard Court
10845 Rancho Bernardo Road
Hollister, CA 95023
Otahal, C. 1998. Wilson's Warbler (Wilsonia pusilla). In The Riparian Bird Conservation Plan:a strategy for reversing the decline of riparian-associated birds in California. California Partners in Flight. http://www.prbo.org/calpif/htmldocs/riparian_v-2.html
Three subspecies recognized: one in the east W. p. pusilla and two in the west W. p. pileolata and W. p. chryseola. In California W. p. pusilla does not occur. However, it is listed as a vagrant to Washington and Oregon (A.O.U. 1957). The form W. p. pieolata breeds in the Warner and White mountains of central eastern California (A.O.U. 1957). The final race, W. p. chryseola is a widespread breeding species in California. It breeds west of the crest of the Cascades and Sierra Nevada, as far south as the San Bernardino mountains (A.O.U. 1957). Both pileolata and chryseola migrate through California. The Wilson's Warbler is very rare in winter in California, but specimens of both pileolata and chryseola have been collected during this season (Dunn and Garrett 1997).
MANAGEMENT STATUS: No special management status.
HISTORICAL BREEDING DISTRIBUTION
CURRENT BREEDING DISTRIBUTION
AVERAGE TERRITORY SIZE
In oak-bay-laurel habitat in Marin County, Stewart (1973) found 24 territories averaging 0.57 ha (1.3 ac) and ranging between 0.2-1.3 ha (0.5-3.2 ac). In willow-scrub habitat in Marin County, Stewart (1973) found territories averaging 0.48 ha and ranging from 0.3 to 1.0 ha. Males may wander up to 300 meters from their normally defended territories (Stewart 1973).
TIME OF OCCURRENCE AND SEASONAL MOVEMENTS
Arrive from Mexico in late April and early May (USDA Forest Service 1994). These birds breed from late April into early August with peak activity in June (USDA Forest Service 1994). At a central coast location, territorial males begin to arrive in mid-March, females arrive one to two weeks later, and pair formation begins an average of 15 days after the arrival of the males (Stewart 1973). In the latitude of the San Francisco Bay area, migrants arrive slightly earlier along the coast of San Mateo County than they do inland in Santa Clara County (Jaramillo pers. obs.). Males appeared earlier and their numbers peaked earlier during spring migration, relative to females, at a migratory stopover site in Santa Clara County (Otahal 1995). Arrive on breeding grounds from late March in the south west, a month to six weeks later in the north (Curson et al. 1994). The subspecies chryseola arrives significantly earlier on territory than pileolata (Dunn and Garrett 1997). Older males appear to return earlier than second-year males and females (Stewart 1973).
Leaves breeding grounds from early August (Curson et al. 1994). The race chryseola is an earlier migrant than the other two subspecies (Dunn and Garrett 1997). Individuals of chryseola may be found in areas away from the breeding grounds as early as July 10, but more typically during mid-August (Dunn and Garrett 1997). Fall migration peaks in late August and early September (USDA Forest Service 1994). Fall migration occurs from 1 August through late October with slight peaks in late August and mid-September in the South San Francisco Bay Area (Otahal 1995). Males and females did not show a differential migration pattern in fall migration at a migratory stopover site in the southern San Francisco Bay Area (Otahal 1995).
Extent of wintering in California:
A winter range map based upon Christmas bird counts is available at the BBS web site and probably should be included in the species account. Wilson's Warblers winter on the coast and in the interior of southern California (Zeiner et al. 1990). They are rare winter residents along the coast and interior Grinnell and Miller 1944). Winter residents are rare in southern California (McCaskie et al. 1979).
Wilson's Warblers are common migrants in almost all woodland and shrub habitat on the coast and in the interior (USDA Forest Service 1994). Wilson's Warblers are common to abundant migrants in the lowlands during the spring and in mountains and lowlands during the fall (Zeiner et al. 1990). Beedy (1975) reports that post-breeding Wilson's Warblers forage along forest edges and within dense coniferous forests. In migration, woodlands with a shrub understory and chaparral are used (USDA Forest Service 1994). Riparian habitat was documented as being utilized as a stopover site where some individuals were making multi-day stays and putting on fat prior to the continuation of their migration (Otahal 1995). Uses all kinds of woodland and tall scrub with undergrowth on migration (Curson et al. 1994). Western birds follow the coast and mountain ranges to Central America (Curson et al. 1994). In migration stays close to the ground within the protecting plant cover (Grinnell and Miller 1944). In seasons of migration, low thick vegetation is preferred but not solely in the vicinity of water (Grinnell and Miller 1944).
Stopover periods were found to be the same (average of 4 days) for males and females during spring migration in riparian habitat located at the south end of San Francisco Bay (Otahal 1995). Mass change at a riparian stopover site in Santa Clara County was the same (average of 0.4 grams) for males and females during spring migration (Otahal 1995).
Forages mainly low in height, usually below 2 m in height (Bent 1953). Wilson's Warblers are known to eat insects gleaned from foliage low in the canopy or understory (USDA Forest Service 1994, Curson et al. 1994). Also are known to flycatch in the canopy (USDA Forest Service 1994, Curson et al. 1994), in fact flycatching is a primary mode of foraging (Bent 1953). Stewart (1973) found individuals ranging 125-300 m (410 -984 ft) from the nest while foraging. Wilson's Warblers often forage at habitat edges (Timossi 1990). Males were frequently observed foraging 10-17 meters from the ground, confirming that this species in not necessarily restricted to low vegetation for its foraging habitat (Stewart 1973). Prefers deciduous over non-deciduous sites for feeding during breeding season (Morrison 1981). In Wyoming willow thickets it feeds most commonly at 0.6 -1.2 meters from the ground but feed at all levels; feeds at slightly higher levels (mostly at 2.4-4.9 meters) later in the summer (Hutto 1981).
Eat insects gleaned from foliage low in the canopy or understory (USDA Forest Service 1994). A small amount of seed, fruits and berries are taken also (Zeiner et al. 1990). Bent (1953) states, based on 52 stomach samples, that animal matter makes up 93% of the food of the Wilson's Warbler, with vegetable matter making up less than 7%. The most common insect eaten (35% of the animal matter) are Hemipterans, mainly Black Olive Scale and leafhoppers. The Hymenoptera come in second (31%), mainly wasps and ants. The Tipulidae (Crane Flies) made up a good number of the prey items and 9% of the animal matter was comprised of assorted species of beetles, mainly Chrysomelidae. Finally, caterpillars made up 5% of the animal matter ingested. The vegetable matter was made up largely of fruit pulp ingested during September and October.
Chosen nesting substrates include shrub or grass (USDA Forest Service 1994) as well as on the ground (Zeiner et al. 1990). In Stewart's (1978) study along coastal California, blackberry (Rubus sp.) vines provided support 74% of the nests. Plant species commonly chosen as the nesting substrate include nettles (Urtica sp.), wild rose (Rosa californica) and ferns (Stewart et al. 1978). In Marin County, the primary nesting substrate was blackberry (Rubus sp.) which provided horizontal runners for support, overhanging leaves for concealment and growth style in of a dense tangle, sometimes so thick that ground predators would find it hard to penetrate (Stewart 1973). Bent (1953) lists Skunk Cabbage as a substrate as well as Azaleas (Rhododendron sp.) in the Yosemite region and Salal (Gaultheria shallon) and Bracken (Pteridium aquilinum) in the Seattle, Washington region.
HEIGHT OF NEST
The height of the nests ranges from 0.3- 3 meters (USDA Forest Service 1994) as well as on the ground (Zeiner et al. 1990). Typically, nests are located at ground level (DeGraaf and Rappole 1995), although occasionally they are found 0.9 meters from the ground (Zeiner et al. 1990). It appears that ground nesting is more common in the more eastern races pileolata and chryseola, while higher nests are more common in the Pacific form chryseola (Dunn and Garrett 1997). Mean height for nests in blackberries is 69 cm (Stewart et al. 1978).
PLANT SPECIES CONCEALING THE NEST
In the Sierra Nevada, the typical nest is at the base of from on to four horizontal willow branches, under which the nest is placed for concealment (Stewart et al. 1978). In Marin county, nests in blackberry are often concealed by other blackberry fronds above the nets (Stewart 1973). Bent (1953) describes a nest which was overhung by Azalea stems. Further concealment is usually provided by a thick growth of perennial herbs (USDA Forest Service 1994).
PERCENT NEST COVER
Often nest is placed at the base of a shrub or in grass hummock for cover (Curson et al. 1994). Often in dense blackberry plants with overhanging leaves concealing the top of the nest (Stewart 1973). Quantitative measures of nest cover are not available, or unknown.
60-100% (Timossi 1990).
AVERAGE TOP CANOPY HEIGHT
The mean height of cover vegetation was estimated to be 15 meters and 5.5 meters in two Marin County study sites (Stewart 1973).
DOMINANT PLANT SPECIES IN CANOPY
Wilson's Warblers find breeding cover in willows, alders and shrub thickets (USDA Forest Service 1994). Bent (1953) states that in the Lassen Peak region of California, the presence of willows and alders appear to be the chief factors influencing habitat choice, rather than any other plants associated with them. The Wilson's Warbler is more abundant in younger (40-75 yr) than older (>105 yr) Douglas-fir stands (Pseudotseuga menziensii) in the Pacific Northwest (Manuwal and Huff 1987). Noted as breeding in the following habitats: Douglas-fir, Western hemlock (Tsuga heterophylla) -Sitka Spruce (Picea sitchensis), Redwood (Sequoia sempervirens), ponderosa pine (Pinus ponderosa), larch (Larix sp.)- white pine (Pinus sp.), Lodgepole Pine (Pinus contorta), fir (Abies sp.)-spruce (Picea sp.), aspen (Populus sp.)/hardwoods, mountain meadows, alpine meadows, subalpine marshes, riparian areas but avoids chaparral and pinyon (Pinus sp.)- juniper (Juniperus sp.) associations (DeGraaf and Rappole 1995). Willows (Salix sp.), alders (Alnus sp.), and dogwoods (Cornus sp.) are also favoured by this species (Grinnell and Miller 1944).
CO-DOMINANT PLANT SPECIES IN CANOPY
The relative density of the cover vegetation, based upon the point quarter method of vegetation analysis, in a breeding site in Marin County was California Bay Umbellularia californica (41%), Coast Live Oak Quercus agrifolia (24%), California Buckeye Aesculus californica (19%), Canyon Oak Quercus chrysolepis (9%) and five other species (7%) (Stewart 1973).
AVERAGE SHRUB COVER
Shrub layer is considered essential for reproduction (Timossi 1990). Quantitative measures of required shrub cover are not available.
DOMINANT SHRUB SPECIES
Most commonly found in dense tangles of blackberry in riparian woodland along the central coast (Stewart et al. 1978). Coyote Bush (Baccharis pilularis), California Sage (Artemisia californica), Coffeeberry (Rhamnus californica) and Thimbleberry (Rubus parviflorus) shrubs make up the majority of the shrub species in a Marin County site (Stewart 1973).
AVERAGE FORB COVER
No quantitative or qualitative measures available.
DOMINANT FORB SPECIES
Poison oak (Rhus diversiloba), blackberries (Rubus sp.), and ferns form thickets favorable to this species (Grinnell and Miller 1944). California blackberry (Rubus californica), nettle (Urtica sp.), Western Sword Fern (Polystichum munitum), Lady Fern (Athyrium filix-femina) and Poison Oak were dominant understory plants in Marine County site (Stewart 1973).
No known attraction to logs.
Nest often well concealed in a grass hummock (DeGraaf and Rappole 1995, Curson et al. 1994).
Nests near water or in wet meadows (Zeiner et al. 1990).
Sapling-sized trees are used for breeding (Timossi 1990).
DISTANCE TO WATER
Prefers to nest near water (Timossi 1990).
An open cup nest (USDA Forest Service 1994, Curson et al. 1994), The nest is a bulky cup of dry leaves, bark shreds, thin dead weed stems, grass blades and stems and is lined with fine dry grasses, rootlets and hair (Harrison 1978, Curson et al. 1994). The form chryseola places the nest between 0.3 and 0.9 meters from the ground, while pileolata and pusilla tend to place their nests on or near the ground (Dunn and Garrett 1997).
Often nest in loose colonies of several pairs (DeGraaf and Rappole 1995, Harrison 1984, Curson et al. 1994). Harrison (1984) reports that breeding birds from the previous season are the first arrivals the following spring suggesting site fidelity. These birds exhibit breeding site fidelity, individuals which have bred in previous years tend to return to territories that they established previously (Stewart 1973).
There are no flight displays in this species. Males advertise by singing from a perch, often semi-concealed from view. Rarely sings higher than six feet from the ground (Grinnell and Miller 1944).
Mostly monogamous although polygyny (bigamy) has been reported (13% of pairs) in the Sierra Nevada (Stewart et al. 1978). The relatively high incidence of polygyny creates problems for accurate counts as the number of singing males may not be cleanly related to the number of breeding females in the area.
The clutch size is of 4-6 eggs with an average of 5 (USDA Forest Service 1994, Curson et al. 1994).
Female only (USDA Forest Service 1994).
11-13 days (USDA Forest Service 1994).
DEVELOPMENT AT HATCHING
Altricial (USDA Forest Service 1994).
10-13 days (Harrison 1978), 8-10 days (Curson et al. 1994), 8-10 days (Stewart 1973).
Both adults feed young but males usually don't begin feeding young until they are three to four days old (Stewart 1973).
NUMBER OF BROODS
Few second clutches were laid after successful fledging of first broods (Stewart 1973). This appears to be more common in chryseola than in pileolata or pusilla (Dunn and Garrett 1997).
Wilson's warblers in the west are common Brown-headed Cowbird hosts, which has resulted in extirpation in some lowland areas (Garrett and Dunn 1981). However, DeGraaf and Rappole (1995) states that Wilson's Warblers are "not normally parasitized by cowbirds". The Wilson's Warbler has been known to successfully raise a cowbird and a warbler nestling in one nest (Friedmann et al. 1985).
Breeds along edges of alpine meadows to 3000 meters (Curson et al. 1994). Altitudes of nesting range from near sea level, as at Point Lobos, Monterey County, up to at least 2900 meters on east slope of Mount Whitney, Inyo County (Grinnell and Miller 1944).
Dunn and Garrett (1997) mention that the Wilson's Warbler has disappeared from most coastal lowlands south of Santa Barbara County. They attribute this disappearance to the elimination of lowland riparian thickets in southern California and the effects of Brown-headed Cowbird (Molothrus ater) parasitism. Because Wilson's Warblers frequent wet meadow and riparian areas with much natural edge, the effects of habitat fragmentation on these birds is unknown and it is possible that habitat variables, such as shrub layers, are more important than fragmentation per se (USDA Forest Service 1994). In fact, fragmentation of forested areas and the subsequent second growth scrub that grows up may be beneficial to Wilson's Warblers. However, local disappearances of breeding populations may be related to increased susceptibility of Wilson's Warblers to cowbird parasitism due to fragmentation of habitat (USDA Forest Service 1994). The effect of cowbird parasitism on the Wilson's Warbler particularly with respects to population decreases has not been adequately studied or published on.
Presumably, the minimum patch size is probably within the range of territory sizes of 0.4-1.2 hectares (USDA Forest Service 1994).
The loss of herbaceous cover, especially during the breeding season, may increase vulnerability to nest parasitism and predation (USDA Forest Service 1994). The effects of disturbance on Wilson's Warblers are largely unknown but their ground-nesting habits and susceptibility to cowbird parasitism suggests that disturbance could have negative effects (USDA Forest Service 1994). Prefers wet clearings in early stages of regeneration (DeGraaf and Rappole 1995). Sensitive to removal of deciduous tree species (Morrison 1981).
ADJACENT LAND USE
Because of Wilson's Warblers susceptibility to Brown-headed Cowbird parasitism, cattle and equestrian staging areas near breeding habitat may have negative impacts.
Prefers moderately cool climate (Grinnell and Miller 1944). Locally the temperature is moderately low and the humidity normally high (Grinnell and Miller 1944).
Dependence on insects for food resources makes this species susceptible to direct effects of insect control measures.
Wilson's Warblers are depredated by accipiters, small mammals, weasels and snakes (USDA Forest Service 1994). Possible predators observed near the nest where the Western Scrub Jay (Aphelocoma californica), the Steller's Jay (Cyanocitta stelleri) and the garter snake (Thamnophis sp.) (Stewart 1973).
DEMOGRAPHY AND POPULATION TRENDS
A total of 53.7% males and 36.8% females (9.5% could not be reliably sexed) mist netted from a central coast population between 1979 and 1995 (Chase et al. 1997).
Estimated annual adult survival of summer residents was 50.3% derived from banding at Point Reyes Bird Observatory (Chase et al. 1997). Male survival was found to be slightly (but statistically insignificantly) higher than females (Chase et al. 1997). Adult survival ranged from 22.2% to 89.1% over a ten year period of time (Chase et al. 1997). Stewart et al. (1978) observed a return rate of 60.8% for color banded males. Based on Stewart et al. 1978: 63 nests examined, 60.3% nests fledged at least one young, 34.9% of nests lost to predation, no nests were paracitized by cowbirds, 4.8% of the nests were lost to adverse weather.
In California, the Wilson's Warbler experienced a 1.6% annual decline between the period 1966 - 1996. This trend was significant (P=0.03). British Columbia and Washington state also showed a decreasing, albeit not statistically significant trend. In contrast, Oregon positive but not significant trend (Sauer et al. 1997).
MANAGEMENT ISSUES AND OPTIONS
Riparian vegetation and wetlands should be retained and restored to manage for this species as it requires riparian or wet meadow sites for reproduction (USDA Forest Service 1994). During the breeding season, nest sites would be susceptible to damage by herbivory or other activities which might remove the dense herbaceous layer required by the Wilson's Warbler. As well, these activities may increase vulnerability to nest parasitism and predation (USDA Forest Service 1994). Water diversions, shrub and willow removal and cattle grazing have been suggested as having negative effects on Wilson's Warblers (USDA Forest Service 1994). Since Wilson's Warblers appear to be highly susceptible to cowbird parasitism, management that results in aggregations of livestock or humans during the nesting season within 5 - 10 km (3-6 miles) of areas managed for nesting Wilson's Warblers should be avoided (Verner and Ritter 1983).
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