Monday, September 25, 2006

Birds of the Mid-Atlantic #24: Swainson's Thrush

Like spring migration, fall migration proceeds in stages, each bearing different sets of species. The birds one sees on the morning after a cold front will be much different in late September than in early September. In the thrush family, most veeries pass through the D.C. area during early September. By late September, most veeries have departed, and the Swainson's thrushes and gray-cheeked thrushes are on the move. Sunday was a good day for spotting Swainson's thrushes, as I saw several of them at the National Arboretum in D.C.

Swainson's thrush is a member of the genus Catharus. Thrushes of this genus are small and brown with some degree of breast spotting. Most Catharus thrushes are noticeably smaller than American robins and slightly smaller than wood thrushes. Similarities in shape, size, and plumage make this the most challenging North American thrush genus, even though it is fairly easy to distinguish Catharus thrushes from other genera in the thrush family.

The most distinctive of eastern Catharus thrushes is probably the Swainson's thrush. The key field marks are on the face of this species. Swainson's has prominent buffy eye rings. A buffy line over the bill connects the eye rings so that the bird appears to wear spectacles. Now one field mark is not enough to base an identification, so make sure to check the back and breast even on bespectacled birds. The back, wings, and tail should be a cold grayish brown with no hint of rufous; from the rear a Swainson's thrush should resemble gray-cheeked more than hermit or veery. The breast should be covered with small dark spots on a buffy background; the spots are less heavy than a wood thrush but bolder and darker than a veery. The spots and buffy wash extend from the throat about halfway down the breast. Put these marks together, and you have yourself a Swainson's thrush.

Swainson's thrushes breed in a wide band across southern Canada and the northernmost states of the United States, including the Appalachians in upstate New York and Vermont. Very few spend the summer at the higher elevations of the Blue Ridge. Though they prefer spruce woods for breeding, Swainson's thrushes can be found in any type of forest and even some edge habitats during migration. The birds we are seeing now are on their way to the Andes of western South America.

Image credits: Top, Photo by NPS; Bottom, Illustration by Louis Agassiz Fuertes.

Crossposted at Blue Ridge Gazette.