(Transcript of today's For the Birds)
One of the abundant backyard birds in north country in October is our little White-throated Sparrow. Juncos are starting to outnumber them at feeders now, and by the time the first snow flies most of them will be further south, but these little stripe-headed guys are right now packing away almost as many seeds as their stripe-backed counterparts in the mammal world, the chipmunks.
White-throated Sparrows come in two color forms, one with bright white stripes on their heads,
and one with tan stripes.
Each color form makes up about 50 percent of the population, and each sex is represented by about 50 percent of each color form. Virtually 100 percent of all mated pairs include birds of opposite color as well as opposite sex—a system called negative assortative mating. Researchers find that the white-striped forms are very aggressive and territorial while the tan-striped birds are more nurturing and not particularly territorial. The human counterpart would be if Scarlett O’Hara always ended up with Ashley Wilkes while Melanie Hamilton always ended up with Rhett Butler.
All this has been known for a long time. But researchers are now finding an intriguing anomaly in White-throated Sparrow chromosomes. In mammals, the sex chromosomes X and Y match up when producing a male, and two X’s match up to produce a female; while in birds, the W and Z chromosomes match up when producing a female and two Z’s produce a male. Although our X and Y chromosomes or the W and Z chromosome counterparts in birds have very few genes in common, they match up during meiosis because they do have a small but critical area in common. In the exceptional case of White-throated Sparrows, another chromosome, Chromosome 2, acts like these sex chromosomes because of a weird phenomenon called a chromosomal inversion. The showy white-striped form birds have the heterozygous form, comparable to our male XY pairing, while the plainer tan-striped birds have the homozygous form like our female XX chromosome pairing. Scientists published a paper last week likening the Chromosome 2 situation to the sparrows having a second set of sex chromosomes, a fascinating possibility for a species with its unique assortative mating system. I think the media made it sound weirder and more sex-charged than it is, but it’s still interesting.
In another recent study, scientists found that male juncos have two spring songs. Birds in an area apparently sort out whose land is whose via their loud territorial song, and then they pretty much ignore each other when singing that tune. But each male also has a softer, more melodic song used to attract a mate. And juncos are very suspicious of every male singing that song and get far more aggressive when they hear it. Apparently they’re more concerned about defending their mate than their property.
When the October wind is rattling my windows and the leaves are piling up, my juncoes are not singing at all, and sparrow genes and chromosomes don’t interest me. These sturdy little beings scratching out their meals under my feeders are a warm and gentle reminder that winter is headed our way. Even as the thought makes me shiver, these warm little birds remind me that even tiny little bodies can survive just about anything Mother Nature can dish out. Thoreau described juncos as “Leaden skies above, snow beneath,” and when a flock takes off, their delicate white tail streamers moving in a flurry like so many snowflakes, I can’t help but remember just how beautiful snow can be.