40: Brown-headed Cowbird

I write to you from Arapahoe, Ute, and Cheyenne land. I am interested in learning about the different animals that live in the place where I was born. I want to mention that biological classification as taught by western science has its roots in racism, sexism, and transphobia – here’s a good explainer about why.

Today’s animal is a fascinating one who by human standards is unbelievably unethical. Do not model your behavior on the Brown-headed cowbird (Molothrus ater), seriously. I mean, some people have – looking at you, Europeans who colonized the Americas – but again, I do not recommend it.

The Brown-headed cowbird is a small blackbird, with a shorter, thicker bill than other blackbirds. The females are brown, while the males have brown heads and the black/iridescent-winged bodies of your typical blackbird. Interestingly, although the bird is named cowbird because they are known to follow cattle herds and feast on the insects flushed from the grass as the cattle graze, they are native to North America and thus probably once followed bison herds instead. Bison herds, of course, were largely confined to the Great Plains, and cowbirds would have stayed there, but once Europeans brought cattle across the Atlantic, the cowbirds spread out wherever cattle live – from coast to coast. Today they live everywhere in the United States, Canada, and deep into Mexico.

The thing about cowbirds that makes this spread a problem is their method of raising – or not raising – their children. This isn’t a 90s style moral panic. Unlike most birds, cowbirds are brood parasites – North America’s most common, in fact! Cowbirds are non-monogamous and lay many more eggs than most birds – up to forty in a season. The female prowls woodlands in search of the nests of other birds, often smaller songbirds, and lays her eggs there, forcing the bird whose nest it is to feed and raise her young. Cowbirds lay eggs in the nests of 220 different species. Fascinatingly, it seems that individual female cowbirds almost always lay their eggs in a nest belonging to a single species. According to the Cornell Lab,

“Some birds, such as the Yellow Warbler, can recognize cowbird eggs but are too small to get the eggs out of their nests. Instead, they build a new nest over the top of the old one and hope cowbirds don’t come back. Some larger species puncture or grab cowbird eggs and throw them out of the nest. But the majority of hosts don’t recognize cowbird eggs at all.

Cowbird eggs hatch faster than other species’ eggs, giving cowbird nestlings a head start in getting food from the parents. Young cowbirds also develop at a faster pace than their nest mates, and they sometimes toss out eggs and young nestlings or smother them in the bottom of the nest.”

With the help of human European settlers bringing cattle and spreading their herds across North America, cowbirds have been implicated in the decline of several bird endangered species. Not great! But of course, we can’t hold cowbirds, or any other non-homo sapiens animal, to our own standards of morality – which is why at least one scientist feels that they have an unfair reputation.

Matthew Louder researches the behavior of cowbirds. He found that mother cowbirds return to lay eggs again in nests where their young have been successful, too, suggesting that they are closely monitoring their young, even if they are not actively raising them. Meanwhile, baby cowbirds somehow learn to be cowbirds, despite growing up in a nest with birds of a different species. Louder and colleagues devised an experiment to figure out how this happens:

“…Researchers attached radio transmitters to adult cowbirds and their young. The data revealed that the juveniles didn’t follow older birds away from the nests and rarely ventured to their mothers’ homes. Instead…The data showed that young cowbirds leave the host nests shortly after sundown and roost overnight in the fields where the species typically lives before returning to their foster families the next day… The team thinks these night flights—which may be spurred by an innate preference for roosting in fields—give the cowbirds some independence from their foster parents and keeps them from becoming something they’re not. Since adult cowbirds roost together at night in the same fields, the young birds’ excursions could also give them the opportunity to mingle with their own species and learn the right behaviors.

Louder thinks his results help show that even parasites have to work hard to survive. “There's this weird animosity towards cowbirds among the public,” he says, but in his mind, it’s not deserved. Not only do the adults have to trick the hosts, sneak their eggs in, and keep an eye on different nests, but the chicks have to put some effort in just to learn to sing, forage and act like cowbirds.

“These guys are really cool. They have these crazy behaviors and what they're doing is really complex,” he says. “If this was easy, everybody would do it.”