Geographic variation in Black-capped Chickadee songs and singing behavior

Songbird species differ in how their songs vary geographically, and the vocal behavior of Black-capped Chickadees (Poecile atricapillus) exhibits an especially intriguing pattern of variation. Throughout most of their range (i.e. from Nova Scotia to British Columbia), males sing a simple two-tone "fee-bee-ee" (with the "fee" slightly higher in frequency than the "bee-ee" and an amplitude break between the ''bee" and "ee''), and each male shifts the pitch of his single song type over a range of about 800 Hz. Birds sing differently, however, on the islands of Martha's Vineyard, Chappaquiddick, and Nantucket off Massachusetts. Island singing differs from mainland singing in four ways: (1) most island songs are monotonal, with the two whistles on the same frequency; (2) island songs exhibit much greater structural diversity than mainland songs; amplitude breaks may occur in the first, second, or both main whistles; (3) island males typically have repertoires of two or more different songs; and (4) song dialects occur both between and within the islands (even on Chappaquiddick, which is only 6 km wide). Song dialects and repertoires of different songs also occur in some Black-capped Chickadee populations in Oregon and Washington. The distinctive singing of birds on Massachusetts' offshore islands, and of birds in Oregon and Washington, may have arisen because these populations are sedentary and isolated. In mainland populations, young chickadees often migrate or irrupt, and the considerable movement of these young birds could promote uniform behavior from coast to coast among mixing populations. Geographic uniformity of song among mainland males is perhaps also maintained by young males learning an "average" song from their sound environment. In contrast, isolated groups of males (as has been shown for young birds of typical fee-bee-ee populations in the laboratory) may express divergent songs and singing behavior.