Ice Age Redstarts
What happened to birds during the last Ice Age? Recent studies of mitochondrial DNA have allowed scientists to peek back in time to find out.
The glacial ice sheets covered up many migratory birds' nesting grounds in Canada and the northern United States, and they had to move south to ice-free areas. The Rocky Mountains or Great Plains served as dividers and isolated populations into eastern and western areas.
Birds such as the yellow warbler, Swainson's thrush, Wilson's warbler, and others, survived in separate ice-free areas in the east and the west for thousands of years, enough time for their DNA to diverge. After the Ice Age they re-colonized the north but there are still recognizable eastern and western subspecies.
The American redstart also survived the Ice Age, but there appears to have been no western area where they persisted. The southeastern United States is the most likely location of their refuge. There are no currently recognized subspecies of the American redstart lending further credence to the one refuge idea.
Interestingly, the birds from Newfoundland are somewhat different in their DNA and there may have been a second Ice Age refuge off the coast of Newfoundland in an area now covered by ocean.
This article summarizes the information in this scientific paper:
Colbeck, Gabriel J., Gibbs, H. Lisle, Marra, Peter P., Hobson, Keith and Webster, Michael S. 2008. Phylogeography of a Widespread North American Migratory Songbird (Setophaga ruticilla). Journal of Heredity, 99(5): 453-463
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