Here’s why sparrows with a whitestriped head make lousy parents
In recent days, one of my favorite winter birds has probably arrived in a backyard near you.
I am referring to the white-throated sparrow, the stout-hearted little brown bird from the north that hangs out under area feeders from now until spring.
One reason I like these sparrows is they actually have white throats, which makes them easy to identify at a glance. Whitethroats also sport a patch of yellow between the bill and each eye, another easy field mark.
Beyond those easy-tospot white throats and yellow lores, things get exceedingly complicated with these sparrows. If you look very closely, you might notice that some have white stripes on top of their heads and others have tan stripes. The plumage difference is not a question of male vs. female. Both sexes come with either head stripe. Rather, the males of one plumage breed almost exclusively with the females of the other plumage.
Here’s why the opposites attract (and I am not making this stuff up): Because of a certain chromosome variation, the birds with the whitestriped heads are promiscuous and lousy parents. The birds with the tanstriped noggins are monogamous and devoted parents.
Because of their superior chick-raising skills, tan-striped males are the mate of choice for both the tan-striped and white-striped females. But because the whitestripe females are more aggressive, they get to pair off with the tanstriped males.
That leaves the whitestriped males to pair off with the tan-striped females, and — given their promiscuity — often more than one. Studies have shown that roughly 95 percent of the whitethroats pair off in this opposites-attract fashion, and that they produce white-striped and tanstriped offspring in equal proportion — thereby maintaining the balance between the two morphs.
Since we see the white-throats in nonbreeding season, their sexual preferences are rarely on display, which may be just as well.
In any event, I’ll bet the late great naturalist Edwin Way Teale would still say: “For me all the frail, enduring beauty of the world finds its voice in the song of the white-throat.”