Wilson's warbler

Wilson's warbler (Cardellina pusilla) is a small New World warbler. It is greenish above and yellow below, with rounded wings and a long, slim tail. The male has a black crown patch; depending on the subspecies, that mark is reduced or absent in the female. It breeds across Canada and south through the western United States, and winters from Mexico south through much of Central America. It is a very rare vagrant to western Europe.

Taxonomy
Wilson's warbler was formally described in 1811 by the ornithologist Alexander Wilson under the binomial name Muscicapa pusilla. The type locality is southern New Jersey. The species was moved to the genus Wilsonia by the naturalist and ornithologist Charles Lucien Bonaparte in 1838. Zoologist Thomas Nuttall moved it to Sylvania in 1840, and by 1845, many authors included it in Myiodioctes. In 1899, the American Ornithological Union returned the species to Wilsonia. The species is currently assigned to the genus Cardellina. The genus name Cardellina is a diminutive of the Italian dialect Cardella, a name for the European goldfinch, and the specific epithet pusilla means "very small".

There are three recognized subspecies:
 * C. p. pusilla was described by Alexander Wilson in 1811.
 * C. p. pileolata was described by German naturalist Peter Simon Pallas in 1811.
 * C. p. chryseola was described by Robert Ridgway in 1902.

The chryseola subspecies, which nests in northern coastal California to southwestern coastal Canada, has a distinctive orange-tinged yellow forehead. The population of the subspecies has declined sharply in the 21st century because it migrates preferentially to the southern end of the Baja Peninsula in Mexico, where luxury resort and residential developments have replaced the bird's habitat.

Description
Wilson's warbler is a small passerine, ranging from 10 to 12 cm in length, with a wingspan of 14 – and a mass of 5 –. It has a plain green-brown back and yellow underparts. The male has a small black cap. Males of the western race C. p. chryseola are greener above and brighter than males of the eastern, nominate race. Individuals from Alaska and the west-central portion of the species' range average slightly larger than those found in eastern and Pacific coastal populations. Its song is a chattering series of loud descending notes. The call is a flat "chuff".

Wilson's warbler resembles the yellow warbler: the latter is readily distinguished by its different shape, yellow wing markings, and yellow tail spots.

Distribution and habitat
The breeding habitat is fairly open woodland with undergrowth or shrubs and thickets in moist areas with streams, ponds, bogs, and wet clearings. Wilson's warbler breeds in northern Canada and the western US; it winters in overgrown clearings and coffee plantations, forest edges, deciduous forests, tropical evergreens, pine-oak forests, mangroves, thorn-scrub, riparian gallery forests, brushy fields, and mixed forests. At all seasons, it prefers secondary growth, riparian habitats, lakes, montane and boreal forests with overgrown clearcuts. It is a very rare vagrant to Western Europe.

Breeding
Nesting generally begins in early March in west coast populations, and extends into August in the northern range. The female does the majority of the nest building. The cup nest is typically constructed of vegetation and lined with grasses and hair. It is often sunken into moss or sedges at the base of shrubs. The clutch varies from 2 to 7 eggs, which are creamy or off-white with fine reddish spots. The young are altricial. The montane populations generally have a higher clutch size and nest success rate than those on the coast. The eggs hatch at 11–15 days and the young fledge at 8–13 days; adults care for them for several weeks. Some montane populations are polygamous (one male breeds with multiple females). Wilson's warbler is a frequent host for the brown-headed cowbird.

Feeding
Wilson's warbler is an insectivore, feeding primarily on insects gleaned from leaves and twigs, or caught by flycatching. Some of these insects include beetles, bees, or caterpillars. Wilson's warbler is an active forager, moving rapidly through shrubs, on the ground, and sometimes in taller trees during the winter. Feeding birds often twitch their tails or flick their wings nervously. The observed feeding rate of the male Wilson's warbler was not significantly different between males with or without mates. It also eats a few berries.

Books

 * Ammon, E. M., and W. M. Gilbert. 1999. Wilson’s Warbler (Wilsonia pusilla). In The Birds of North America, No. 478 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.

Thesis

 * Ammon EM. Ph.D. (1995). Reproductive strategies and factors determining nest success in subalpine ground-nesting passerines. University of Colorado at Boulder, United States, Colorado.
 * Hagar JC. Ph.D. (2004). Functional relationships among songbirds, arthropods, and understory vegetation in Douglas-fir forests, western Oregon. Oregon State University, United States, Oregon.
 * Hedges LK. M.S. (1997). Passerine migration in the Davis Mountains of Trans-Pecos Texas: An analysis of bird banding data for August 1992-May 1997. Sul Ross State University, United States, Texas.
 * Otahal CD. M.S. (1996). Sexual differences in Wilson's Warbler migration. San Jose State University, United States, California.
 * Ramos Olmos MA. Ph.D. (1983). SEASONAL MOVEMENTS OF BIRD POPULATIONS AT A NEOTROPICAL STUDY SITE IN SOUTHERN VERACRUZ, MEXICO. University of Minnesota, United States, Minnesota.
 * Santana-Castellon E. Ph.D. (2000). Dynamics of understory birds along a cloud forest successional gradient. The University of Wisconsin - Madison, United States, Wisconsin.

Articles
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 * Benson A-M, Andres BA, Johnson WN, Savage S & Sharbaugh SM. (2006). Differential timing of Wilson's Warbler migration in Alaska. Wilson Journal of Ornithology. vol 118, no 4. p. 547-551.
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 * Chase MK, Nur N & Geupel GR. (1997). Survival, productivity, and abundance in a Wilson's warbler population. Auk. vol 114, no 3. p. 354-366.
 * Clegg SM, Kelly JF, Kimura M & Smith TB. (2003). Combining genetic markers and stable isotopes to reveal population connectivity and migration patterns in a neotropical migrant, Wilson's warbler (Wilsonia pusilla). Molecular Ecology. vol 12, no 4. p. 819-830.
 * Eckhardt RC. (1979). The Adaptive Syndromes of 2 Guilds of Insectivorous Birds in the Colorado USA Rocky Mountains. Ecological Monographs. vol 49, no 2. p. 129-150.
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 * Gilbert WM & Carroll AF. (1999). Singing in a mated female Wilson's Warbler. Wilson Bulletin. vol 111, no 1. p. 134-137.
 * Gram WK & Faaborg J. (1997). The distribution of neotropical migrant birds wintering in the El Cielo Biosphere Reserve, Tamaulipas, Mexico. Condor. vol 99, no 3. p. 658-670.
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 * Kelly JF. (2006). Stable isotope evidence links breeding geography and migration timing in wood warblers (Parulidae). Auk. vol 123, no 2. p. 431-437.
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 * Kelly JF, Delay LS & Finch DM. (2002). Density-dependent mass gain by Wilson's Warblers during stopover. Auk. vol 119, no 1. p. 210-213.
 * Kelly JF & Hutto RL. (2005). An east-west comparison of migration in North American wood warblers. Condor. vol 107, no 2. p. 197-211.
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 * Komar O, O'Shea BJ, Peterson AT & Navarro-Siguenza AG. (2005). Evidence of latitudinal sexual segregation among migratory birds wintering in Mexico. Auk. vol 122, no 3. p. 938-948.
 * Lemon RE, Struger J & Lechowicz MJ. (1983). Song Features as Species Discriminants in American Warblers Parulidae. Condor. vol 85, no 3. p. 308-322.
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 * Morrison ML. (1981). The Structure of Western Warbler Assemblages Analysis of Foraging Behavior and Habitat Selection in Oregon USA. Auk. vol 98, no 3. p. 578-588.
 * Morrison ML & Meslow EC. (1983). Bird Community Structure on Early Growth Clear Cuts in Western Oregon USA. American Midland Naturalist. vol 110, no 1. p. 129-137.
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 * Pittaway R. (1989). Behavioral Identification of the Wilson's Warbler. Ontario Birds. vol 7, no 1. p. 28-29.
 * Pomara LY, Cooper RJ & Petit LJ. (2003). Mixed-species flocking and foraging behavior of four Neotropical warblers in Panamanian shade coffee fields and forests. Auk. vol 120, no 4. p. 1000-1012.
 * Raley CM & Anderson SH. (1990). Availability and Use of Arthropod Food Resources by Wilson's Warblers and Lincoln's Sparrows in Southeastern Wyoming USA. Condor. vol 92, no 1. p. 141-150.
 * Rappole JH, King DI & Barrow WC Jr. (1999). Winter ecology of the endangered Golden-cheeked Warbler. Condor. vol 101, no 4. p. 762-770.
 * Rhymer JM, McAuley DG & Ziel HL. (2005). Phylogeography of the American woodcock (Scolopax minor): Are management units based on band recovery data reflected in genetically based management units?. Auk. vol 122, no 4. p. 1149-1160.
 * Richardson TW, Pyle P, Burnett R & Capitolo P. (2003). The occurrence and seasonal distribution of migratory birds on Southeast Farallon Island, 1968-1999. Western Birds. vol 34, no 2. p. 58-96.
 * Rubenstein DR & Hobson KA. (2004). From birds to butterflies: animal movement patterns and stable isotopes. Trends in Ecology & Evolution. vol 19, no 5. p. 256-263.
 * Ruth JM & Stanley TR. (2002). Breeding habitat use by sympatric and allopatric populations of Wilson's Warblers and Yellow Warblers. Journal of Field Ornithology. vol 73, no 4. p. 412-419.
 * Scott A. (1994). Atlanta area winter record for Wilson's warbler. Oriole. vol 59, no 2-3. p. 65-66.
 * Skagen SK, Melcher CP, Howe WH & Knopf FL. (1998). Comparative use of riparian corridors and oases by migrating birds in southeast Arizona. Conservation Biology. vol 12, no 4. p. 896-909.
 * Smaldon R. (1990). Wilson's Warbler New to the Western Palearctic. British Birds. vol 83, no 10. p. 404-408.
 * Spicer GS. (1978). A New Species and Several New Host Records of Avian Nasal Mites Acarina Rhinonyssinae Turbinoptinae. Journal of Parasitology. vol 64, no 5. p. 891-894.
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 * Stewart RM, Henderson RP & Darling K. (1977). Breeding Ecology of the Wilsons Warbler in the High Sierra-Nevada California. Living Bird. vol 16, p. 83-102.
 * Swanson DL, Carlisle HA & Liknes ET. (2003). Abundance and richness of Neotropical migrants during stopover at farmstead woodlots and associated habitats in southeastern South Dakota. American Midland Naturalist. vol 149, no 1. p. 176-191.
 * Wade P, Smith T & Girman DJ. (1996). Molecular genetic analysis of wintering and breeding populations of Wilson's Warbler (Wilsonia pusilla), a neotropical migrant passerine. Bulletin of the Ecological Society of America. p. PART 2) 464, 1996.
 * Weicker JJ & Winker K. (2002). Sexual dimorphism in the birds from southern Veracruz, Mexico, and other localities. III. Wilson's Warbler (Wilsonia pusilla). Journal of Field Ornithology. vol 73, no 1. p. 62-69.
 * White JD & Gardali T. (2004). Low incidence of cowbird parasitism on Swainson's thrushes in central coastal California. Western Birds. vol 35, no 3. p. 156-167.
 * Yong W, Finch DM, Moore FR & Kelly JF. (1998). Stopover ecology and habitat use of migratory Wilson's warblers. Auk. vol 115, no 4. p. 829-842.
 * Zajac RM, Cerasale DJ & Guglielmo CG. (2006). The rapid response of plasma metabolites to changes in feeding rate in a small passerine Wilson's warbler Wilsonia pusilla. Journal of Avian Biology. vol 37, no 4. p. 405-408.