SHREVEPORT, La. (KTAL/KMSS) – We’re highlighting the White-throated Sparrow (Zonotrichia albicolis) during the month of December, and chances are you host White-throated Sparrows in winter if you have a bird feeder in Louisiana.
The bird remains the most common North American sparrow species at most feeders in the winter from Texas to the East Coast, where you can readily hear them sing their song that sounds like, “Ooooo Canada, Canada, Canada.”
Sadly, their numbers have dropped precipitously in recent years. Recent trend data from
Cornell’s eBird.org shows that in the KTAL viewing area, the numbers of White-throated Sparrows
have dropped as much as 50% since 2007.
It may seem odd to focus on sparrows for something as obviously glamorous as a monthly bird
article. Sparrows are usually mostly brown, most don’t sing during the winter (White-throated being an exception), they’re not flashy, they’re pretty secretive, and they’re sort of the generic “bird.”
In fact, many birders have a catch-all name for sparrows when we see a group of them flush or
fly away unidentified: Little Brown Jobs or LBJs.
So to get right to the intrigue of the world of White-throated Sparrows, let me throw this at you:
They have four sexes. You can check that again, but it’s not a typo. They. Have. Four. Sexes.
So how does a bird have four different sexes?
All White-throated Sparrows have a very obvious stripe over the eye, running from the base of the bill, over the eye, to the back of the head. This stripe is called a “supercilium” or “eyebrow stripe,” and it’s a common feature of many birds. But in White-throated Sparrows, the stripe isn’t always the same. In some individuals, it’s white; in others, it’s tan. And this happens to males and females. So to be clear, you have white-striped males, tan-striped males, white-striped females, and tan-striped females–so why is this interpreted as four sexes?
There are many resources to support this claim, but let me quote from episode 300 of a podcast called “Distillations” that’s supported by the Science History Institute and hosted by Sam Kean, a best-selling science book author.
Speaking on published research, Kean writes, “In over 99 percent of cases, tan-striped birds would mate only with white-striped ones. You almost never have tan-tan or white-white couplings…Now, there’s no perfect definition of biological sex, but one definition involves what proportion of the population a typical individual can and will reproduce with. In virtually all animal species, that’s ½, or one over two. Which is why we have two sexes.
“But in white-throated sparrows, there are tan-headed males, tan-headed females,
white-headed males, and white-headed females. And each group reproduces with only ¼ of
the population, or one over four. In other words, there are four separate sexes.”
So, yes, the species is still separated as either male or female. But consider that if, say, a white-striped male will only mate with a tan-striped female, then he ignores white-striped females just as he would ignore other males. Biologically speaking, from the male white-striped bird’s
perspective, tan-striped females are a separate sex than white-striped females. And vice versa
from the female perspective.
So, yeah. Four sexes.
Before we answer why this occurs, first understand that, although uncommon, there are similar
occurrences in the animal kingdom.
There’s a Eurasian shorebird called a Ruff that has three male sexes; each of the three differs completely in plumage, and one of the male sexes has plumage that is actually identical to females.
Clown Fish are actually all born male, and any male has the ability to become a dominant female when the first female dies.
But back to the White-throated Sparrows. Why are they so complicated? I mean, isn’t it already tough enough with two sexes?
It turns out that when scientists researched the species’ DNA, they found that one of the chromosomes had a sequence of over 1000 genes “flipped” in the white-striped birds.
Among the genes affected? Head stripe color.
But that’s not all.
Turns out genes that affect aggression got flipped in the white-striped birds, too. White-striped individuals are more aggressive, they sing more, they’re more territorial, they’re more promiscuous, and they have poor parenting skills.
So to balance out all their personality issues, they either have to take Xanax or they have to…wanna guess?
Yeah, they have to mate with tan-striped birds.
That’s why the birds create white-tan pairs instead of “same-stripe” pairs. Basically, a genetic mutation created four sexes that can be defined by behavior and appearance in only one species. This creates the possibility of an evolutionary advantage because White-throated Sparrow parents have opposing traits, with one tending to be aggressive and protective and the other tending to be nurturing and attentive.
And you can probably look outside any winter morning and see them hopping around, blending in with all the normal, boring birds (if there is such a thing as normal.)
John Dillon is the former president of Louisiana Ornithological Society, a regional reviewer for Cornell University’s www.ebird.org and is on the Louisiana Bird Records Committee. He sits on the Board of Directors of the Briarwood Caroline Dormon Nature Preserve, teaches English Literature at Minden High School, and is the founder of the Minden High School Nature Club.