By Rick Borchelt
All winter long, white-throated sparrows (Zonotrichia albicollis) are the most common sparrows at my backyard bird feeders.. They usually show up in late October and keep mostly to the brush pile or dense shrubs, unless they are on the ground picking up the small seeds that other birds kick out of the hanging feeders. As the winter days lengthen toward spring, they’ll begin singing their distinctive Grand-Old-Canada-Canada-Canada refrain that will serve them so well, attracting mates and defending their territories in the boreal forests, where they spend their summers. By May, most of them have moved north until the following fall.
With their boldly striped heads and bright throats, these sparrows are distinctive yard visitors, and sometimes a dozen or more are scratching under my feeders. I rarely see them up on the hanging platform feeder or the squirrel-busting seed tube, though, ground feeders that they are.
White-throated sparrows are so common I take them for granted most days. When I walk around Lake Artemesia, I can easily spot 100 or more. And even if you don’t see them, you’ll hear their high-pitched tseet calls, especially toward dusk as they find their nighttime roosts.
Courtesy of Cephas/Creative Commons
The flock of sparrows in my yard is a mix of birds with bright white head stripes and throats, and some with tan head stripes and darker throats. I shrugged off the color differences between the whites and tans for years. Even my 1940’s era Peterson Field Guide to Birds noted simply that the tan birds were immatures that had not yet grown into their adult plumage. It takes a couple years for bald eagles to acquire their characteristic white head, too.
A Canadian ornithologist working with the birds on their breeding grounds in the early 1960’s figured out that white-striped birds are white-striped for life, and tan-striped birds are tan-striped for life. And this isn’t related to gender;about half of all males of the species have white-striped heads and half have tan-striped. Same holds true for the females.
Courtesy of Cephas/Creative Commons
Curiously, field naturalists noted that when it comes to settling down with a partner, white-striped sparrows of both genders choose a mate of the opposite color. The mystery behind this assortative mating pattern started to unravel in 2016, when scientists did a deep dive into the genetics of white-throated sparrows. They noticed that there is a major genetic difference between the two forms, or morphs. The tan-striped morphs have normal chromosomes, white-striped birds have a large scrambled section of one of their chromosomes.
This scrambled genome does more than dictate the color of the stripes on the birds’ heads. White-striped males are super-aggressive, sing louder and actively bully tan-striped males on their breeding grounds. Female white-stripes are also a bit on the assertive side. While it’s usually the male birds that sing in the avian world, white-striped female sparrows sing, too. Male and female tan-stripes, in contrast, both seem to live more low-key lives.
Following common wisdom, we might expect that the female of this species of both color morphs would choose hypermasculine white-striped males for partners. But in practice field observers found otherwise. Turns out that tan-striped males make better mates, choosing better nesting sites and helping more with feeding hungry nestlings when they hatch.
White-striped males and females both seem to be more interested in chasing other birds than enjoying domestic bliss. Tan-striped males and females are the epitome of homebodies, faithful partners, and good neighbors: quiet, calm, and intent on the business of raising the kids. You need a tan-striped sparrow in the mix to have a good household!
In practice, this curious behavioral anomaly ends up creating a sparrow species with functionally four sexes — male white-striped, male tan-striped, female white-striped and female tan-striped. And the mate-selection rules are quite rigid: More than 90% of all these sparrows follow the white-with-tan dating rule.
What happens when the birds break the color barrier? As you might expect, two white-striped birds together argue and irritate each other — a lot. Two tan-striped birds together just don’t get a lot accomplished, and they often have difficulty maintaining their nest territory and protecting their young. You need the best of both for nesting success.
You can see this behavior playing out under the feeders in your backyard, too. White-striped birds rule the playground; tan-striped ones hang back. As they start to sing in March and April, you’ll notice the white-striped birds start earlier in the season and sing more often.
This curious quadrisexual arrangement is, as far as we know now, unique in the animal world. It just shows how the most amazing animal lives play out right under our noses — or at least under our bird feeders. While the deep mysteries of the lives of the plants and animals that share our Earth so often elude our understanding, sometimes all it takes is a keen eye and some time in the field with them to plumb the complicated secrets of their lives.
Have questions for Rick about the world of nature in and around the city, or suggestions for future College Park Wild columns? Drop him a note at rborchelt@gmail.com.
