Back in the spring, while I was driving across the USA, when I got to the eastern side of South Dakota, to Sioux Falls, I found that I was about half way across the continent. And over the next few days it became obvious that there were some new birds turning up. The eastern meadowlarks were behind me and western meadowlarks were ahead. Eastern bluebirds were back there and western ones were ahead. The swifts switched from chimney swifts to white-throated swifts. Lark buntings and Brewer’s blackbirds were beside the road, and the mix of birds really did change.
It’s quite a striking thing, and I’d been told that I would notice it by the National Audubon Conservation Director, Greg Butcher. Of course it’s particularly noticeable if you are travelling day by day in the same direction as I was.
I was talking to Ian Newton at the Bird Fair about this and he pointed me towards his excellent book The Speciation and Biogeography of Birds for the answers. What appears to have happened is that when the glacial ice sheet spread south across North America some species were probably wiped out but others found refuges on both the eastern and western sides of the current USA. When the ice retreated the species spread from their glacial refuges. Sometimes the passage of time had allowed enough differences to evolve that the birds rushing from their glacial refuges no longer interbred and they met somewhere near the middle of the continent and banged up against each other in the area where the East/West divide is still noticeable. Others probably ‘met in the middle’ and simply interbred and merged. In some cases this sort of thing happened with races which initially had been part of the same species and in other cases probably with different but similar species.
It’s reassuring to know that there is a simple explanation for this phenomenon, and I probably ought to have been able to work it out for myself, but I didn’t. What it shows is that your day’s birding in America is influenced by the last ice age of about 18,000 years ago. 18,000 years is just a blink in the age of the Earth (4.6 billion years) and the time since life evolved here (3, 600, 000, 000 years).
- Posted in: USA trip