Laura Erickson's For the Birds

Monday, September 10, 2007

White-throated Sparrows

I love this time of year, when the ground beneath my feeders is alive with White-throated Sparrows. They’re the chipmunks of the bird world—striped-headed, blending in with the ground, and feeding on the same seeds and bugs.

White-throated Sparrows are famous for their song, a clear, whistled “Old Sam Peabody” or “Oh, sweet Canada.” Even though the song is heard during migration, it’s sung much more frequently once they reach their breeding grounds, and is one of the defining characteristics of the north woods, at least in early summer. Right now, few birds are singing at all, and when one tries, the song just peters out.

The White-throated Sparrow is unique in one characteristic. The species has two different plumages—white-striped and tan-striped. At hatching, half of all males will be tan-striped and half of all females white-striped. So the species is not sexually dimorphic. Rather, the colors are like human hair color, with one fascinating difference. Brunette humans do not exclusively select blonds for mates, and blonds don’t exclusively select brunettes. But with White-throated Sparrows, over 96% of all pairs have one bird of each type. With White-throated Sparrows opposites attract, a phenomenon called negative assortative mating.

Obviously, birds do not have access to mirrors, so how do they know what type is the opposite, to know what to look for when seeking a mate? They don’t base their selection on appearance but on behavior. The chromosomal difference between the tan-striped and the white-striped individuals causes a behavioral difference. White-striped birds of both sexes are very aggressive and dominant, and tan-striped birds of both sexes are very nurturing. In particular, tan striped females are extraordinarily good parents, and white-striped males are extraordinarily good at obtaining and defending a quality territory. But white-striped individuals are all so aggressive that male and female white-striped birds bicker too much to form a pair bond. And tan-striped individuals are so unaggressive that two of them have problems obtaining a territory. In nature, they’re behaviorally disposed to select the opposite form.

Interestingly, in captive studies, females of either color prefer the less aggressive tan-striped males, and males of either color prefer the feistier white-striped females. Based on that, you’d think that most successful pairs in nature would have a white-striped female and a tan-striped male. But that combination is found only half as often as the opposite. Even though 50% of all babies of both sexes are each color, apparently white-striped males and tan-striped females survive better, probably because of the superior nurturing skills of the mothers and the superior territorial defenses of the fathers. And apparently white-striped males and tan-striped females both live longer in nature, perhaps because males win fights and females avoid fighting altogether.

Trying to keep it all straight makes my head swim, rather like the ground beneath my feeders right now, squirming with dozens, and some days hundreds, of White-throated Sparrows. These birds, as fascinating to think about as they are lovely to listen to and watch, make our backyards as entertaining as any movie or TV show. My cats can hardly tear themselves from my windows these days, and I’m often standing right there with them, filled with wonder and joy. White-throated Sparrows materialize wherever we toss sunflower seeds or white millet—the point where migration and miracles and magic converge.

(This is the transcript of today's For the Birds radio program. Listen here. When you subscribe to the iTunes podcast, your iTunes will show the transcript on the lyrics page and include a photo of that day's bird. My podcasts are absolutely free.)

For more information about White-throated Sparrows, check out the Birds of North America entry, with details about their negative assortative mating and unique chromosomal situation. (By subscription, and worth every penny!) You can also read the Cornell Laboratory of Ornithology's All About Birds entry. This isn't quite as detailed but has a wealth of information and is free.