Calls of familiar bird species sound quite different in the Arizona woodlands than they do in the Minnesota backyard, and there’s a good reason for that — the birds themselves are different. Take White-breasted Nuthatches (WBN) for example.
In the eastern U.S. and Canada, we hear a nuthatch advertise its presence with its familiar, nasal “yaankk, yaankk” call. But in more western forests, the call is fainter, faster, more repetitive, and higher — more like “yirr, yirr, yirr”. Now this may be entirely too nerdy for consideration here, but the birds even look different. Can you spot the differences?
Look at the amount of black on the top of the head, and the color in the feathers that cover the wings (called wing coverts). The eastern, Carolina form of the WBN has black edging on the feathers that cover the wings; the Rocky Mountain form has none of that. The eastern form has a much wider black stripe on the top of its head, and less white between the black stripe and its eye than the Rocky Mountain form.
So what? Well, these are the kinds of differences, including the differences in the call, that allow birds to discriminate more carefully about who they will or won’t mate with during the breeding season. And there aren’t just two forms of WBN, but likely as many as four, according to the latest poll by ornithologists in the know.
A key geological event that formed the basin and range regions of western North America is probably responsible for the differentiation of subspecies (forms) of many bird species, like the White-breasted Nuthatch. Three distinct forest regions developed during glacial cycles in the U.S. and Canada in the past 14 million years, resulting in eastern (east of the Rockies), interior west (Rockies to the Sierras) and Pacific coastal (Sierra crest to the Pacific Coast) forests. Geographic separation and then specializations by interbreeding populations in these regions have produced the distinct differences in the birds.
The differences might seem slight to some of us, but to the birds (and to the ornithologists that catalog them), they may be enough to separate them into three or four distinct species.