Genetic evidence for widespread population size expansion in North American boreal birds prior to the Last Glacial Maximum

Pleistocene climate cycles are well known to have shaped contemporary species distributions and genetic diversity. Northward range expansions in response to deglaciation following the Last Glacial Maximum (LGM; ~21,000 years ago) have been surmised to lead to population size expansions in terrestrial taxa and changes in seasonal migratory behaviour. Recent findings, however, suggest that some northern temperate populations may have been more stable than expected through the LGM. We modelled the demographic history of twenty co-distributed boreal-breeding bird species of North America from full mitochondrial gene sets and species-specific molecular rates. We used these demographic reconstructions to test how species with different migratory strategies were affected by glacial cycles. Our results suggest that effective population sizes increased in response to deglaciation during the middle Wisconsin period (~45,000 years ago) whereas genetic diversity was maintained throughout the LGM despite shifts in geographic range. We conclude that earlier glacial cycles prior to the LGM have most strongly shaped contemporary genetic diversity in these high-latitude species. We did not find differences in historic population dynamics between species differing in migratory behaviour, contributing to growing evidence that major switches in migratory strategy during the Last Glacial Maximum are unnecessary to explain contemporary migratory patterns.