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Wilson's Warblers

by Mike Scully

This article first appeared in the October 2003 issue of the San Antonio Audubon News.

I had gotten out at first light, looking to put in a quick thirty minutes in the park before hurrying to work. Mid-September is an excellent time for birding around here but I seem to have hit a lull, twenty-five minutes of intense scrutiny of trees, undergrowth and pond had yielded no migrants. Then, as I was walking back to the car, the morning sun broke through the clouds. At that very moment, a little bird burst from the dense brush to my right. Slim and rather long-tailed, its underside flashed bright yellow in the sunlight as it described a swift arc over my head and plunged into the bushes on my left. As often happens around here when migrants are moving through, a sprightly little Wilson’s Warbler (Wilsonia pusilla) had brightened my day.

Wilson’s Warblers are active little birds, frequently darting to intercept flying insects or hovering while gleaning small arthropod prey from foliage. They tend to remain fairly close to the ground, often foraging in undergrowth or dense brush, the tail jauntily cocked upwards like a wren. Adding to their appeal is a tame and confiding nature, often permitting the careful birder a close approach.

All Wilson’s Warblers are plain olive above shading to brighter yellow below. Various shades of yellow and/or olive plumage can be confusingly common among our migrant warblers. Adult male Wilson’s wear a distinctive black cap. Female and immature Wilson’s outnumber adult males in Fall and, if no traces of a cap are evident, these are best distinguished from similar species by their small size, large dark eye set in a yellow face with a brighter yellow ‘eyebrow’ above, and by their unmarked dark tails.

The breeding range of this species extends across the north woods of Canada from Newfoundland to the Pacific and north across the Northwest Territories and Alaska. In the lower 48, they breed on the West Coast as far south as Northern California and in the Rockies as far south as New Mexico. The males defend small breeding territories in brushy areas, frequently near water. A single brood raised by a monogamous pair is the norm, but second broods are common in California and polyandry has been observed in some mountain populations.

Three subspecies occupy different areas of the breeding range. The brightest subspecies (W. p. chryseola) occurs on the West Coast. Far more probable in Texas are representatives of the drabber eastern subspecies (W. p. pusilla) and the intermediate-plumaged Rocky Mountain and Alaska form (W. p. pileolata). Determining exactly to which of these latter two subspecies a particular Wilson’s belongs can be problematic; suffice to say the brightest birds we see are likely pileolata, while the predominantly olive individual I saw just yesterday was probably pusilla.

Wilson’s Warblers winter in a variety of brushy habitats from Panama north through Mexico. Most birds winter south of the United States but some (primarily W. p. pusilla) can be found in winter along the Gulf Coast. Scattered individuals have been reported on local Christmas counts. Most Wilson’s migrate via an overland route, hence the seasonal abundance of this species as they pass through Texas. A few are known to migrate directly across the Gulf of Mexico. In addition, strays have turned up in fall as far offshore as Bermuda.

In October of 1985, after what must have been an epic of survival subsequent to a New England hurricane, an adult male Wilson’s was spotted on the southwest coast of Britain (Cornwall). Sadly, that intrepid little bird likely rested, refueled, and then struck out boldly for distant Mexico.

 

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