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"Partly because of his unseasonable dalliance, this tiny golden fragment becomes one of the most conspicuous of migrants..." -Dawson

Identification
These are small warblers (3 3/4") that are yellow underneath and olive above. Males sport a distinct black cap which may be mottled or absent in females. Wilson's Warblers have long dark tails without any white or yellow patches.

Subspecies
Of three recognized subspecies of Wilson's Warblers, only one, W. p. chryseola has ever been caught at the lab. The distinguishing characteristic for this subspecies is its bright yellow forehead with an orange tinge. This subspecies breeds from coastal southwest British Colombia to southwest California. W. p. pileolata breeds in eastern California north to Alaska and is larger and without an orange tinge. W. p. pusilla breeds in Alberta and eastern Canada, and is also larger than our local subspecies.

Range
This warbler's breeding range extends as far north as northern Alaska and spans across Canada from British Colombia to Newfoundland and Maine. It also breeds along the west coast, in the Sierra Nevadas, and the Rocky Mountains. The winter range is primarily northern Mexico south to Panama but some birds will winter in south Texas, Louisiana, southern California, and Arizona. Most of the chryseola birds winter from northwest Mexico south to Panama but some have been found wintering in southern California and Arizona.

Song
The song of our local birds is a series of staccato notes which build in volume towards the end. The notes are rather harsh sounding and have been likened to "chi chi chi chit Chit Chit Chit".

Banding History
Wilson's Warblers are the most frequently caught bird at the lab; sometimes as many as 1000 birds may be captured in one year. The birds start to arrive at Andrew Molera State Park in March and peak activity is in the month of May. Some birds pass through on their way northward, while others will stay to breed and can be found readily through the summer. The last birds to pass through this area in the fall are likely to be seen in October. Over the years more than five thousand of these warblers have been banded at the Big Sur Ornithology Lab.

The Big Sur Ornithology Lab will be analyzing its banding data for this species to detect (if any) trends in survival and productivity for this species at Andrew Molera State Park.

Habits
These are active warblers which sally and glean insects from bark. They eat mostly insects and spiders but also consume berries in summer and fall. On their summer grounds, Wilson's Warblers can be found primarily in riparian areas with low shrubs and dense ground cover. They may breed in coniferous forests or moist broadleaf woodlands. During migration they may be found in a variety of deciduous woodlands and thickets. These birds are tame and inquisitive. They nest on or near the ground, forming a cup nest of twigs, stems, and leaves.

Status
The Wilson's Warbler is not a threatened or endangered species. Data from the Breeding Bird Surveys suggests no broad trends for the pusilla or pileolata subspecies: there are local increases in parts of their range, declines in other parts. In California, the coastal chryseola subspecies has disappeared from most lowlands south of Santa Barbara County. Its decline has been attributed to the loss of riparian thickets in that region accompanied by an increase in brood parasitism by Brown-headed Cowbirds.

A coalition of public and private organizations called Partners in Flight (PIF) is trying to reverse the population declines of many North American Songbirds. The California chapter of PIF has produced a document called the California Riparian Habitat Conservation Plan, which outlines suggestions for restoring California's riparian bird species by preserving and actively restoring riparian habitats and reducing the impacts of human industry and brood parasitism. The Plan is completely voluntary.

The Wilson's Warbler is a focal species in this Plan, that being one of a few selected indicator species whose populations may be intensively monitored to assess the health of a particular ecosystem. Economy of effort is the rationale behind monitoring a few carefully selected focal species: if the ecosystem is largely healthy, the populations of many more unmonitored species are probably healthy as well.

- Edwin Price and Lionel Leston

References

VWS Featured Birds:
Townsend's Warbler
Yellow Warbler
Flycatcher
Warbling Vireo
Wilson's Warbler