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Wilson's Warbler
Cardellina pusilla
– Family
Authors: Ammon, Elisabeth M., and William M. Gilbert

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Adult male Wilson's Warbler, Ohio, May.
Figure 1. Distribution of Wilson’s Warbler.
Adult female Wilson's Warbler, California, April

Editor’s Note: Phylogenetic analyses of sequences of mitochondrial and nuclear DNA indicate that two species formerly placed in Wilsonia (canadensis and pusilla) are more closely related to the Red-faced Warbler (Cardellina rufifrons) and other Neotropical warbler species than to the only other species formerly placed in Wilsonia (citrina, now in Setophaga). See the 52nd Supplement to the AOU Checklist of North American Birds for details. Future revisions of this account will reflect these changes.

First described by naturalist Alexander Wilson, this wood-warbler is a familiar and characteristic species of wet habitats in the boreal and Pacific coastal zones. The most widespread in its genus, Wilson’s Warbler breeds throughout Alaska, most of Canada, and south through the western United States to southern California and New Mexico, often the most abundant warbler in these regions. Distinguishable from similar species by its black cap, simple and bright color pattern, characteristic tail-waving, and short, chattering song, this species is quickly recognized darting through shrub willows and undergrowth. It nests on or near the ground at elevations ranging from sea level to the alpine zone. Breeding territories are usually located in riparian habitats or wet meadows with extensive deciduous shrub thickets.

Just as it breeds across a wide altitudinal and latitudinal range, Wilson’s Warbler also winters in a great diversity of environments, ranging from the east to the west coast of Mexico and Central America, and from the Caribbean lowlands to the cloud forests of the continental divide. Moreover, it is the only migrant warbler regularly found in tropical high plains (paramo). In the winter, Wilson’s Warbler shows a diversity of behavioral strategies, with many birds defending territories, others acting as winter “floaters,” and still others joining large mixed-species foraging flocks.

Breeding Wilson’s Warblers are socially monogamous, but polygyny occurs and a high rate of extrapair paternity has been found in one population. Brood parasitism by Brown-headed Cowbirds is generally low, although it can be locally high in some areas. Extensive studies have addressed the species’ breeding biology (Stewart 1973, Stewart et al. 1977, Ammon 1995, R. Bereson, J. Rhymer, and R. C. Fleischer, unpubl., WMG), survivorship (Chase et al. 1997), migration (Yong et al. 1998), and diet (Raley and Anderson 1990), but many interesting aspects of its biology remain to be learned. For instance, geographic and altitudinal variation in life history traits, ecophysiology, mating system, extrapair paternity, and cowbird parasitism could be subject to comparative investigations. Also, the evolutionary ecology of above-ground vs. ground nesting Wilson’s Warblers (Martin 1988) and of Wilson’s Warbler winter territoriality deserve additional study.

Long-term trend analysis indicate recent population declines, particularly in the western portion of the species’ range; large-scale destruction of riparian habitat is likely a leading cause. These populations declines and their causes merit further investigation, since declines appear to have become more widespread in recent years.