Friday, December 6, 2013
By Herb Wilson
The fall migration of birds continues in high gear. Most of the leaf-gleaning insectivores like warblers, vireos and tanagers have departed for warmer climes although a few yellow-rumped warblers and palm warblers will linger for a few more weeks.
Seed-eating migratory birds can be more leisurely. Until snow accumulates, these seed-eaters (granivores) can find sufficient food. Sparrows are common granivores and are the main songbird migrants passing through Maine in October. Many of these birds are passage migrants, breeding to our north and passing through Maine on their southward seasonal journey.
Dark-eyed juncos have increased in numbers in the past few weeks. Chipping sparrow numbers have passed their peak. Diligent searching of sparrow flocks will often reward a birder with a view of a Lincoln's sparrow among the more common song sparrows and white-throated sparrows. Swamp sparrows are common now too although you may have to flush them out of fields or marshes to see them well.
A treat this time of year is to see white-crowned sparrows. These large sparrows belong to the genus Zonotrichia, the same genus to which white-throated sparrows belong.
Both of these Zonotrichia species have prominent black streaks on the head alternating with white or tan stripes. Both species have gray breasts without streaking (except for the streaked juveniles of both species).
As the names suggest, the white-throated sparrow has a brilliant white throat, bordered with a thin dark stripe on the lower side. The white-crowned sparrow has a throat that is lighter in color than the gray breast, but never light enough to be called white. There is no black border to the throat in the white-crowned either.
One other feature that can be used to distinguish these two species is the color of the lores, the small feathers between the base of the bill and the eye. In the white-throated sparrow, the lores are yellow. The lores are dark in the white-crowned sparrow.
In both species, the light head stripes may be either white or tan. Birds of either species in their first-winter plumage (the plumage that replaces the juvenile plumage in the fall of the first year) show tan stripes. Adult white-crowned sparrows always have white stripes on their crowns. White-throated sparrows may have either tan or white stripes on the crown as adults. So it's a snap to age a white-crowned sparrow by the color of the head stripes.
All white-crowned sparrows migrating through Maine are passage migrants. In other words, they winter to our south and breed to our north and do not breed in our state. The only time we see them is when they move through on their spring and fall migrations.
In eastern North America, the closest breeding populations are in northwestern Newfoundland, and the northern portions of New Brunswick and Newfoundland. The breeding distribution is wider in western North America, extending south from Canada into the United States in the Rocky Mountains and along the Pacific coast all the way to southern California.
Some ornithologists claim that the white-crowned sparrow is the best-studied songbird in North America. The species is suitable as a research subject because of its abundance, its wide geographic distribution and its fearless behavior as ornithologists make observations.
The song type of white-crowned sparrows varies markedly across its breeding range. Studies on its vocalization have contributed much to our understanding of song learning in songbirds.
Other studies on white-crowned sparrows have demonstrated the remarkable navigation abilities of birds. Birds wintering in the San Jose region of California were captured and carried via plane to Maryland and Louisiana. Upon being released, the displaced birds found their way back to their Alaska breeding grounds in the summer and reappeared at their San Jose wintering areas the following winter.
Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at: