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).
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Remarkable , both for highlighting the wonder of evolution and survival of species, and how relatively quickly we are destroying them. I have a sneaky feeling that some species will even survive us, even if it’s just crows and blue bottles!
It is interesting that northern populations of eastern meadow lark migrate in winter to central america but western meadowlark do not. How this can be explained in evolutionary terms is another mystery.
By the way Ian Newton’s book can be bought much cheaper (-£20) on other web sites
DavidH – that migrationary difference is very interesting – thank you.
What is more remarkable is that a self styled rent-a-mouth for the conservation lobby should drive a car across the USA! Something about people living in glass houses?
Brian – thank you. What a lovely first comment. You are right about the carbon implications of my trip to the USa and these were flagged on my blog at the time. And I am not a self-styled rent-a-mouth – that’s what you call me not what I call myself so it’s hardly ‘self-styled’ is it? Please try to be more accurate on your return. And ‘rent-a-mouth’ suggests that my mouth can be paid for – in fact it works pretty well for nothing!