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Tuesday, 3 August 2010

More on HOUSE FINCH movements

The migratory behavior of HOUSE FINCHES is conspicuous at outer coastal sites where they are absent or scarce as breeders, such as Fire Island, New York and Block Island, Rhode Island. Beyond that, this species is known to be prone to long-distance dispersal (= vagrancy) independent of seasonal migration. Dick Veit’s paper on the westward colonization of eastern and central North America by the birds naturalized in the New York area (Dispersal, population growth, and the Allee Effect: Dynamics of the House Finch Invasion of eastern North America, The American Naturalist 148: 255-274, 1996) is a classic in the scientific study of vagrancy. This study showed that the observed rate at which House Finches spread across North America could not be explained by models using average dispersal distances but depended critically on long distance dispersal events (vagrancy). In addition to demonstrating that House Finches undertake long distance movements, this study further suggests that individuals predisposed toward long distance movements are probably over-represented in the recent ancestry of eastern North American House Finches, perhaps having been rewarded demographically for pioneering previously unoccupied sites.

Although it seems very unlikely that a House Finch would survive an unaided trans-Atlantic crossing, a ship-assisted crossing seems perfectly plausible to me. If I understand descriptions of its subsequent movements in Britain (moving around and even crossing water barriers), these would seem more consistent with those of a wild-born bird than a cage bird. I think we sometimes give cage birds a bit too much credit for being able to survive and regulate in the wild and give wild birds too little credit for being able to get up in the air and out over unexpected horizons (Shai Mitra, New York)