I was back in Arkansas last month, partly on family business, partly revisiting some favorite places in the Ozarks. Things have changed since I lived there. The great homogenizing forces of commerce and mass culture have been at work. You exit the freeway into outposts of Generica: Barnes & Noble, Old Navy, Starbucks. Krispy Kreme, having leapfrogged from the Southeast to the West Coast, is about to colonize Arkansas. There are signs of demographic shifts: more Mexican restaurants, and a couple of Vietnamese sandwich shops in Little Rock.
The natural world has changed too. We never saw armadillos when I was a kid; now they seem to have displaced the possum as the dominant species of roadkill. My cousin Lillian up near the Missouri border has dillo divots in her lawn. Blame it on climate change, or creeping Texanization.
And all the bird feeders I happened to watch during my stay had, along with the cardinals and chickadees, a steady stream of house finches. We never had house finches either; I saw my first here in California. Now they’re found all over the South, and through most of the eastern and central states.
How they got there is an interesting story. Back when native birds had less legal protection, some species became popular as cagebirds. The attractive red plumage and vigorous song of the males made house finches a natural for the pet trade. Trapped in California, they were marketed as “Hollywood finches.” Inevitably, some escaped or were released.
By the 1940s house finches of California origin were nesting on Long Island. They fanned out from there in all directions, adjusting to local climates and sustained in part by bird feeders. A lone pioneer reached Arkansas in 1971, two years after I arrived in California. Within 15 years they had been confirmed as breeding there, and now they’re as ubiquitous as Baptists.
Sixty years is an eyeblink in the life of a species. Eastern house finches don’t show a lot of genetic variation, which may make them more vulnerable than native birds to infectious diseases. In the mid-90s, feeder observers began to notice sick house finches. They were suffering from conjunctivitis caused by the bacterium Mycoplasma gallisepticum, previously known only from domestic poultry. Infected birds either starved to death or succumbed to predators, and the eastern population dipped significantly. But the native purple finches and goldfinches appeared unaffected.
I haven’t followed the Mycoplasma story in recent years, so I don’t know whether the eastern house finches are beginning to evolve a resistance. But it wouldn’t surprise me. According to a study by Jeremy Egbert and James Belthoff at Boise State University, they’re showing other microevolutionary changes, in both anatomy and behavior.
In its western homeland, the house finch is a sedentary bird. But at least some eastern populations have begun to migrate. Migratory behavior in songbirds tends to be pretty inflexible (unlike geese, which learn—and can therefore unlearn—their migration routes). But you can see how the propensity could spread through a population. A finch that gets the hell out before snow blankets the ground is going to leave more descendants than one that freezes solid in a New England winter.
It’s the apparent linkage between form and function that I find interesting, though. Egbert and Belthoff trapped house finches around Boise and at Ithaca, New York, and measured their wing dimensions and other vital statistics. The New York finches had wings that were thinner and more pointed than those of the Idaho birds. The primary feathers—the flight feathers between wrist and wingtip—were shorter near the wrist, longer at the tip.
So? The authors point out that this thin-and-pointed shape is common in migratory birds, for good aerodynamic reasons: a pointed wingtip reduces drag in flight. This holds both between and within species. When a species has both sedentary and migratory populations, the migrants tend to have longer, pointier wings.
What this means is that as the migratory habit has spread among eastern house finches, so has a wing shape that makes long-distance flight easier. Egbert and Belthoff, citing E. O. Wilson, the godfather of Sociobiology, think the behavior likely changed first, the physical structure second.
This doesn’t mean the eastern birds are necessarily on their way to becoming a distinct species. California and New York finches could still interbreed and produce fertile offspring, if they ever got together. But it’s a great example of the routine workings of natural selection, the kind of fine-tuning Peter Grant discovered in his long-term study of the Darwin’s finches of the Galapagos Islands. As recounted by Jonathan Weiner in The Beak of the Finch, Grant found that the beak size and shape of his subjects tracked the availability of seeds of different sizes and hardnesses, which was in turn driven by El Niño events and other climatic shifts. Selection kept varying the mix to produce an adaptive optimum.
It’s all about which physical or behavioral type leaves the most descendants. Sixty years equates to 60 house finch generations—not long enough to evolve, say, iridescent feathers, but long enough for adaptations to a different climate to show up.
What happens when those selective pressures are relaxed, though? If global warming means milder winters in New York, will the Ithaca finches revert to the western norm? Stay tuned.