Origin and Evolution of the Eastern House Finch Population

Aldrich, John; Weske, John

The House Finch (Carpodacus mexicanus) became established in the eastern United States after liberation in the New York City area, spreading from there along the Atlantic seaboard. It has been assumed that the liberated birds were from California. Specimens of the new eastern population resemble most closely those from western California in size but differ from them in having relatively smaller legs and feet. In color, eastern birds resemble House Finches from northeastern Colorado and southeastern Wyoming, as well as those from eastern Washington and northern Idaho. On the basis of historical and size evidence we conclude that the eastern House Finch population is descended from California stock but has differentiated from it in color and size after liberation in the east. The change took place rapidly, as two specimens taken 9 and 11 yr after introduction showed the differences from California birds that now characterize the eastern population. The case of the House Finch parallels that of the House Sparrow (Passer domesticus) in its rapid evolutionary change after introduction and spread into new environments. Several climatic and associated vegetational differences from California were encountered by eastern House Finch pioneers. The lower temperatures have not selected for larger size in the new eastern population as was the case in House Sparrows that moved into colder climates, and no other environmental factor or combination of factors including substrate aspect is the obvious cause of differentiation of eastern birds from their California ancestors. Received 7 February 1977, accepted 20 September 1977.

Origin and Evolution of the Eastern House Finch Population

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