The House Sparrow: Little Troopers

There’s an army in the bushes. You may not have noticed, but there’s a good chance it's nearby, assembling on a sidewalk or conducting drills under the tables of an outdoor cafe. There’s no need to be alarmed—these soldiers aren’t hostile. But they’re as regimented as any human army, and with better-looking uniforms to boot.

I’m talking about House Sparrows. Yep, those chirping little brown birds that were brought over from Europe in the 19th century and eventually squirmed into what seems like every American city block and street corner. Because they're non-native and pretty ubiquitous, House Sparrows don’t get much respect from birders, and are even considered pests. (At least they have it better than their relatives in China, Eurasian tree sparrows; back in the ‘50s they landed themselves on Mao's bad side and became the object of a bizarre eradication scheme that totally backfired.) But while their charms may not be obvious to the casual observer, House Sparrows deserve some credit, if only for their fascinating social structure—one that’s organized, to borrow an analogy from David Attenborough’s Life of Birds series, much like a military unit.

House Sparrows are very social creatures, feeding and roosting in large flocks. As any college freshman can attest, living in close quarters tends to bring out the worst in people, and the same principle applies to birds. Like rowdy undergrads, birds fight over food or the most desirable mates. But constant fighting isn’t good for anyone, animal or human, so many species develop hierarchies within social groups to avoid such conflicts. A troop of gorillas answers to the silverback. Southern elephant seals have a “beachmaster” that controls a harem of females. For House Sparrows, a male's place in the hierarchy is spelled out by the size of the patch of black feathers on his chest—a status badge of sorts that lets everyone know who's in charge and who isn't.

All male House Sparrows have a patch, but the larger the patch, the higher the ranking. It’s all very orderly and every bird knows his place, from the generals to the colonels to the lowly privates. As always, with status come perks: High-ranking males eat at safer food sites. They have bigger and better breeding territories. They’re more dominant in winter flocks, putting them first in line for food. They get more and better mates, and pair up earlier in the year than males with smaller markings.  It’s good to be at the top.

So what determines the size of a bird's patch? Scientists are still trying to figure that out. The size of the patch doesn’t seem to be directly tied to testosterone level, which often drives similar ornamentations in other species (testosterone levels do correlate to the color of the bird’s bill, though). In some animals, ornamentation is a reflection of strength—for example, a moose has to be pretty strong to hold up a huge set of antlers. But House Sparrows don’t need to be strong to show off their large breast patches. What we do know is that badge size does correlate closely with fighting ability, and it also correlates pretty well with age.

Clearly there remain mysteries to be solved. House Sparrows are among the most studied birds on Earth, but their dynamics are still bearing out. For example, though the chest badge factors in to breeding success, so does the size of a male’s white wing patches, which, believe it or not, signal a stronger resistance to lice. And while drabber in plumage, female House Sparrows are the real powerhouses in the flocks, aggressively dominating during certain seasons and getting their pick of the patchiest males.

Even if we don’t totally understand how it works, the sparrows sure do. Next time you see them, instead of walking right past these little birds, take a moment to scan their ranks. Maybe even throw ‘em a salute. 

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