The Fascinating and Complicated Sex Lives of White-throated Sparrows

Could this be the world’s most interesting bird? Sure, it doesn’t look that interesting. In fact, at a glance, it seems like a run-of-the-mill sparrow.

It doesn’t live in far-off exotic places, either: It may be outside your window right now. The White-throated Sparrow is common and familiar, hopping on the ground under bird feeders all over the eastern states in winter. It appears by the hundreds during migration in places like New York City’s Central Park and Chicago’s lakefront parks. But this seemingly ordinary backyard bird has a secret identity—or, actually, four secret identities. And it's these multiple personalites that place the White-throat at the center of mysteries scientists are still working out.

Watch a flock of White-throats in spring and you’ll notice they have two kinds of head patterns. Some wear snappy stripes of black and white across the top of the head. Others have more modest head stripes of dark brown and tan. That superficial difference might not seem like a big deal, but it reflects a remarkable divergence in the lifestyles of these individuals.

For years it was assumed that tan stripes indicated a young White-throat. As late as 1947, in his classic Field Guide to the Birds, Roger Peterson described the adult’s “striped black and white crown” and said the immature was “duller, but with the same essential recognition-marks.” By that time, there’d been hints already that the colors might not be just a function of age. For example, in The Birds of Massachusetts in 1929, ornithologist Edward Howe Forbush mentioned a two-year-old banded White-throat that “had not attained full adult plumage.” He added that “Some require an extra year, or possibly even more, to assume highest plumage.” But he didn’t go so far as to suggest that some adults would never develop those white stripes.

Finally, in 1961, a landmark study by Canadian ornithologist James Lowther revealed the true nature of the color differences. A White-throated Sparrow, he demonstrated, would be white-striped or tan-striped for life—these were permanent color morphs, and about half of all males and half of all females fell into each color type.

It’s not rare for a bird species to have color morphs. Check your field guide: There are morphs in Eastern Screech-Owls, Reddish Egrets, Red-tailed Hawks, and others. But White-throated Sparrows are different. As Lowther discovered, mated pairs of White-throats almost always involved one bird of each color morph: Either a tan-striped male with a white-striped female, or a white-striped male with a tan-striped female. Intrigued, Lowther extended his research, joined by biologist J. Bruce Falls and others. As studies continued, it became obvious that these two color morphs, existing side by side throughout the species’ range, are stunningly different from each other in behavior.

Within each gender, white-striped birds are more aggressive while tan-striped birds are more nurturing. That seemingly simple generalization is based on a vast amount of research. For many years, Falls and his students at the University of Toronto carried out highly detailed studies of White-throated Sparrows, showing how behavioral differences between the morphs touched every aspect of these birds’ lives.

Consider their defense of breeding territories. White-throated Sparrows nest in coniferous and mixed forests of Canada and the northeastern states, but within this habitat, white-striped males tend to be in slightly more open areas, while tan-striped males are often in denser forest. As with most songbirds, they defend their territories mainly by singing. White-striped males sing far more often than tan-striped males. White-striped females also sing fairly often, but tan-striped females rarely do. They all sing variations of the whistled Oh, sweet, Kimberly-Kimberly-Kimberly!, but the songs of tan-striped birds average slightly lower-pitched, and these lower sounds may carry better through dense habitat.

Now, when singing isn’t enough, White-throats will actively chase away intruders on their territory. White-striped males, though, are much more aggressive than tan-striped males in these chases. White-striped females will also take part in chasing, but tan-striped females don’t seem to do much to defend the territory.

In the actual process of raising young, tan-striped birds shine. Only females (of either morph) build the nest and incubate the eggs, but after the eggs hatch, both parents bring food for the young. On average, females bring more food than males. But within each sex, tan-striped birds bring more food than white-striped ones.

So are females most strongly attracted to the tough, macho, white-striped males? Actually, no. Lab studies have found that females of either morph prefer the tan-striped males. White-striped females, more pushy than their tan-striped sisters, grab the tan-striped bachelors right away, so these pairs form more quickly than the opposite combination. Males of both morphs tend to prefer the white-striped females, but those females quickly hook up with tan-striped males if they can, so eventually the leftover birds will form pairs consisting of white-striped males and tan-striped females. Once they’re paired up, tan-striped birds of either sex tend to be faithfully monogamous, but white-striped birds are sometimes promiscuous.

Talk about a complicated dating scene! Nesting pairs consist of one bird from each morph more than 95 percent of the time, but it’s especially interesting to consider what happens on those occasions when two birds of the same morph pair up. In short, it’s likely to be a bad idea. Pairs consisting of two white-striped birds may fail to raise any young because they spend too much time fighting with each other and not enough time feeding the kids. Pairs consisting of two tan-striped birds might fail completely at defending their territory, but that’s a guess; we don’t have much data.

So the morphs differ in traits that parallel the usual differences between the sexes in birds. 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.

We’ve known since the late 1960s that the two color morphs differ in their chromosomes. Tan-striped birds have two identical copies of chromosome 2, but in white-striped birds, one copy of chromosome 2 has a large section inverted, as if it had been put in backwards. Recent work by biologist Elaina Tuttle and others has established that this section of the chromosome is not just inverted, but scrambled in a variety of ways. And it doesn’t just control the color of head stripes. Many different genes here are tightly linked to form a “supergene,” so that birds of one color morph also inherit a whole range of behaviors. The resulting effect is that the White-throat really does operate as a bird with four sexes. For anyone curious about the scientific background, you can read all the technical details here.

As far as we know, there’s no other bird in the world with this unique arrangement. And the lab work to discover the genetic details was done only because we already knew something weird was going on—because people had taken time to watch the behavior of White-throated Sparrows in the wild. Who knows what other mysteries might be just waiting to be revealed? Most of us will never discover anything quite so astonishing as this, but we can all keep in mind that even the most common bird may have its own rare secrets.