The glacial advances in the Pleistocene era have a significant effect on the distribution of animals in North America. The ranges of most birds were most likely shifted southward during the glacial advances. This would have resulted in birds that were forced south into smaller isolated populations. The location of these populations are usually referred to as refugia. As the glaciers retreated the birds reclaimed their former range but retained genetically distinct due to their isolation with southern populations generally showing more genetic diversity than northern populations. This genetic diversity is most often seen is distinct subspecies.

The Gray Jay (Perisoreus canadensis) inhabits the boreal forest ranging from Newfoundland to Alaska. There are generally considered to be 14 subspecies in two or three, depending on the source, morphological variants (Pacific, Rocky Mountain and Taiga).

A study was conducted were mitochondrial DNA of Gray Jay populations from various locations throughout the range of the bird was a analyzed. This analysis demonstrated that the northern most populations had the greatest genetic diversity. This is not consistent with the hypothesis that the Gray Jays  were pushed south as glaciers advanced and than expanded north as the glaciers retreated. This suggests that Gray Jay populations survived in larger numbers in the north perhaps in refugia along the southern edge of the ice sheets. After the retreat of the ice sheets the new northern territory was likely invades via multiple routes including from the northern refugia resulting in the greater genetic diversity in the northern populations. In contrast the southern populations were likely the result of populations being restricted  to smaller geographical areas with smaller populations.

References

Van Els, P., Cicero, C., Klicka, J. 2012 High latitudes and high genetic diversity: Phylogeography of a widespread boreal bird, the gray jay (Perisoreus canadensis), Molecular Phylogenetics and Evolution, 63, 456-465