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 publication:
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
Genetic analyses for many widespread North American species have revealed significant east-west differentiation, indicating that many survived through the Pleistocene in 2 glacial refugia—1 in the eastern and 1 in the western part of the continent. It remains unclear, however, whether other areas may have served as important glacial refugia. Moreover, many such species exhibit widespread genetic similarity within eastern and western regions because of recent expansion from small refugial populations, making it difficult to evaluate current-day levels of gene flow. In this study, we used mitochondrial DNA (mtDNA) control region sequence and amplified fragment length polymorphism markers to survey genetic variation in a widespread migratory bird, the American redstart (Setophaga ruticilld). mtDNA analyses revealed a pattern that contrasts with that found for most other widespread species studied to date: most redstart populations across North America appear to have spread out from a single glacial refugium, possibly located in the southeastern United States, whereas populations in far-eastern Canada may have survived in a second glacial refugium located on the now-submerged Atlantic coastal shelf off the coast of Newfoundland. A pattern of isolation by distance in mtDNA suggested some constraints on current-day gene flow among extant redstart populations. This study thus reveals a recent evolutionary history for this species that differs from that of most other widespread North American passerines and provides evidence for limited gene flow in a species with potentially large dispersal distances.
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