Every once in a while you read something new that changes the way you think about some aspect of our fascinating natural world. For me, a good example was a link my wife Jodi forwarded to me the other day.
She spends way more time browsing the vastness of Twitter than I, and she regularly comes across little nuggets that she thinks I’d like to read. After 40 years together, she knows me pretty well, and she scored a direct hit with a blog post from Audubon, by Ken Kaufman, about the fascinating sex lives of white-throated sparrows.
They, of course, are my favorite sparrow, with their “oh Canada-da-da” song that for many of us is as much a part of the North Country avian lexicon as the call of the loon or the croak of a raven. For years, I, and apparently many others, had assumed that all adult white-throated sparrows sported those bright black and white head stripes, yellow eyebrows, and prominent white throat. If you’ve paid attention, however, you’ve undoubtedly noticed that some white-throated sparrows sport a more subdued plumage, with only a buffy hint of the stripes and eyebrows found on other birds. I had assumed these were immature birds, but as researchers have since discovered, they are actually a color variant, and the adults of this variety will never achieve the splashier colors I long associated with adult birds. It turns out that each variant makes up about half the population, and that they are divided roughly equally between males and females.
That’s interesting, but it’s the rest of the story that’s really worth telling.
It turns out that there are distinct behavioral differences between the two variants as well. The two variants live side-by-side throughout their range, although their behavior is remarkably different. As you might guess, the flashier black and white-striped birds are the extroverts. They sing much more frequently as they defend their breeding territories and both the male and females of this variant both sing in territory defense. And when singing isn’t enough to ward off a rival, they’re much more likely to get physical about it than their buffy-colored cousins.
While a stronger defense of territory would seem to make the black and white striped birds more fit as breeding partners, it turns out that buffy birds make more attentive parents, who bring more food to the nest than their flashier neighbors.
That may be one reason why mated pairs of white-throated sparrows are almost always comprised of one of each color variant. Females of either variant prefer the buffy males, because they make better dads, (they’re also more likely to be faithful), so most of the available buffy males get paired to black and white striped females, because they’re more aggressive in pursuing their attentions. The males of both variants tend to prefer the black and white striped females, which are more helpful in defending territory, but since most of the flashier females have already paired up with buffy males, it typically leaves the very unaggressive buffy females and black and white striped males left to pair up.
This is why as many as 95 percent of white-throated sparrow pairs that researchers have tracked were comprised of one member of each color variant. And in the five percent of cases in which two members of the same color variant mate, there’s usually trouble— particularly if it’s two of the flashier variety. They frequently spend too much time defending territory, or fighting between themselves, to provide for their young. Failed nesting is a frequent result.
Kaufman notes that the differences between the different variants are more typical of the differences between the sexes in most birds. He writes: “Looking at white-throats in the breeding season, we see four distinct types. To oversimplify, we could call them super-aggressive males, more nurturing males, somewhat aggressive females, and super-nurturing females. It’s almost as if the white-throated sparrow has four sexes. That may sound like a joke, but it’s actually a good description of what’s going on.”
These differences, not surprisingly, are genetically based, notes Kaufman. The buffy birds have two identical copies of chromosome 2, but in the black and white striped birds, one copy of chromosome 2 has a large section inverted. It turns out that the mutations in this gene not only determine the color of head striping, but also provide for a whole range of behaviors. “The resulting effect is that the white-throat really does operate as a bird with four sexes,” writes Kaufman, who adds that scientists have yet to find any other bird in the world with an arrangement like this.
Genetic mutations, which drive evolutionary change, are hardly unusual. Yet in most cases, a mutation that provides a survival advantage tends to become dominant in a population. In the case of the white-throated sparrow, the mutation seems to provide an advantage, but only when paired with a member of the population that doesn’t share it.
I know one thing for sure. I’m going to be watching our white-throats a lot closer when they arrive in the North Country once again, sometime in April. Who knew they had such a fascinating story to tell?