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close-up of male shiny cowbird

Brood parasites like this shiny cowbird are master criminals.


These parasitic birds act just like human criminals

An aspiring criminal sneaks past a family home, formulating her plan to break in during the dim hours before dawn. But this isn’t a typical criminal, nor a typical crime. For one, she’s a bird. And instead of stealing valuables, she’ll be leaving something precious behind. A new study lays out just how two devious cowbird species plan and execute the perfect crime, and sometimes even return to the scene after the deed is done.

Rather than doing the hard labor of raising their own chicks, cowbirds and other so-called brood parasites lay their eggs in the nests of various unsuspecting songbirds, which then raise the foreign chicks as their own. It’s a delicate job, and it requires precise timing. Too early, and her egg will stand out in an empty nest; too late, and her chick might not get the attention it needs from its surrogate parent.

Cowbirds, scientists figure, must scope out nests beforehand to get the timing right. “We kind of know it goes on,” says Jeffrey Hoover, an avian ecologist at the University of Illinois in Urbana who wasn’t involved with the study, “but we never had a good data set to point to.” So a team of researchers led by Romina Scardamaglia, a behavioral ecologist at the University of Buenos Aires, decided to use new radio-tracking technology to snoop on two kinds of devious cowbirds in Argentina.

All 41 tagged birds checked their target nests a few days ahead of time to fine tune their timing, the team reports this month in Behavioral Ecology and Sociobiology. Screaming cowbirds (Molothrus rufoaxillaris) were by far the more dedicated, visiting nests an average of 27 times each—almost twice as often as shiny cowbirds (M. bonariensis). That’s likely because the victims of screaming cowbird parasitism have wisened up. They lay their eggs at unpredictable times, so their cowbird parasites have to be especially obsessive about their own timing.

Each of the screaming cowbirds later returned to the scene of the crime, too—one even went back to the same nest 39 times after laying. The shiny cowbirds, on the other hand, tended to avoid nests after they parasitized them. That makes sense in light of their grisly methods: They have a habit of poking holes in all of the eggs they come across, so avoiding a nest they’ve already laid in is the easiest way to avoid accidentally killing their own chicks.

For some victims of cowbirds, these assaults take a toll. The threatened saffron-cowled blackbird (Xanthopsar flavus) in Argentina has declined in numbers partially because of the shiny cowbird’s vicious egg destruction. And in the United States, at least two threatened species have been hit hard enough by brown-headed cowbirds (M. ater) to spark cowbird-killing efforts by conservationists.

Those North American cowbirds are perhaps the most hardened criminals of all the cowbirds, practicing what scientists call “mafia behavior.” “If the host throws out the cowbird’s egg, the cowbird will destroy the nest again and again,” says William Feeney, an evolutionary biologist at the University of Queensland in Brisbane, Australia.

It might be easy to vilify brood parasites, but humans are exacerbating their impacts. In the United States, for instance, cowbird populations have exploded thanks to the easy food and nesting opportunities offered by sprawling patchworks of forest and agricultural land. “Cowbirds are just doing what they’ve evolved to do,” Hoover says. “But humans make it easier for them to do it.”