If you travel abroad, you might find familiar birds such as western wood pewees and cliff swallows wintering in South America. You can even find familiar birds in Australia, such as Caspian terns, barn owls and osprey. This might be unexpected, but birds have incredible mobility.
It is a little more surprising to find some of the more sedentary species, such as great gray owls, horned larks and black-billed magpies in Europe. They may not migrate great distances, but if you trace their ranges around the northern hemisphere, they form a ring from western Europe through Russia and on into North America.
For something really unusual and perplexing, visit the French island of Corsica in the Mediterranean Sea. Take a hike up into the pine forests of the mountains, and you will find many wonderful European birds. There will be hoopoes, firecrests and chaffinches. Among these exotic species you may also see a small nuthatch climbing headfirst down the trunks of the trees looking for insects in typical nuthatch fashion. It has a bluish-gray back, a black cap, a white line above the eye, and a cream-colored to reddish breast and belly. If this description sounds like our red-breasted nuthatch, you would be right.
Until quite recently, the nuthatch on Corsica was considered the same as our red-breasted nuthatch. The range included the coniferous forests of Canada and the western United States plus the island of Corsica and nowhere else. Disjunct populations such as these fascinate scientists. DNA analysis now tells us the form in Corsica is sufficiently different to be regarded a separate species named the Corsica nuthatch, but they are the closest of relatives.
Digging a little deeper into the story, there is a third form closely related to both. This is the Chinese nuthatch which lives in northeast China and the Koreas. This hardly helps. The three forms are each separated by thousands of miles.
Investigating still further, there are two more relatives just a bit more distantly related.
These include the Algerian nuthatch, restricted to the high mountains in northern Africa, and Krüper's nuthatch of Turkey and nearby areas. Together the five forms make up what biologists call a "superspecies," a group of similar forms that shared a common ancestor in the relatively recent past.
This collection of closely related species tells us something about climate change. During the Ice Age, coniferous forests stretched more or less continuously from northern Africa and Europe east through Russia across the Bering Strait and into North America. This forest was occupied by the ancestor of the present day red-breasted nuthatch and relatives.
As the Ice Age ended and the earth warmed, forests retreated northward and up into the mountains, leaving remnants in places like Algeria, Turkey and Corsica. Now isolated, the nuthatch that inhabited the once continuous forest diverged into the five forms we see today. Despite the differences in DNA making them distinct species, the red-breasted nuthatch of North America and the Corsica nuthatch still look almost identical.
Someday I hope to travel to Corsica to see for myself just how similar their nuthatch is to the one now visiting the feeder in my backyard.
Stewart Janes is a biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.