Anis that slip extra eggs into other nests end up with as many surviving chicks as honest anis
Cheating pays, sort of. But for a glossy blue-black bird with a bright yellow eye, cheating doesn’t outdo regular honest parenting.
The greater ani, a type of cuckoo found from Panama to the Amazon Basin, usually starts out as a dutiful parent. Two or three male-female pairs typically build and fill a communal nest “like a big basket of eggs,” says behavioral ecologist Christina Riehl of Princeton University.
But if a snake or some other disaster kills the young, a bereft female sometimes gets sneaky. She slips into neighboring ani nests and leaves an egg here and there that she won’t care for, but the rightful nest owners might. Not all females from trashed nests do that. Some just wait for the next breeding season, when all the birds get a fresh start building another nest.
Greater anis’ sporadic cheating offers a rare chance to compare the success of egg-sneaks with honest mothers in the same species. Over 11 breeding seasons, Riehl and colleagues determined the parentage of more than 1,700 eggs and found 65 eggs in foster nests.
Mothers that parasitize other nests in this way seem to lay more eggs a year, on average, Riehl says. “It’s actually kind of hard to be a parasite,” she says. But the average number of chicks that survived to flutter out of the nest on their own frantic wing power was about the same for all females, Riehl and Princeton colleague Meghan Strong report online February 27 in Nature. The mothers that always cooperated averaged about one fledgling a year, and so did the females that laid stealth eggs.
Birds that off-load the work of raising their chicks by sneaking eggs into some other family’s nest have long fascinated biologists. Figuring out who sneaks and who doesn’t can provide insight into the balance of forces that promotes cheating or cooperation, says behavioral ecologist Dai Shizuka of the University of Nebraska–Lincoln.
Stealth egg-laying’s most dramatic form, which targets other species’ nests, has evolved at least three times in the diverse family of cuckoos and at least four more times in other birds that aren’t closely related, such as cowbirds and some ducks. The greater ani (Crotophaga major) targets its own species. Birds like this are hard to spot but may represent a huge chunk of nest parasites, perhaps 1 percent of all birds, says Rose Thorogood of the University of Helsinki.
Riehl studies greater anis along waterway edges in Panama. When the anis chorus as a group, “it sounds like an outboard motorboat or an eggbeater,” she says. She spent several years figuring out how to use forensic DNA tools to sample the surface of freshly laid ani eggs for traces of maternal DNA that link an egg to its mom. She identified eggs of 210 females and pinpointed 33 birds that cheated after a nest disaster.
The cheaters tend to repeat their deception if disaster strikes them again. Finding individual birds with a tendency to cheat could be “step one,” Riehl says, to testing for some heritable genetic basis for sneaking eggs.
Researchers often view this parasitic egg-laying and cooperative nesting as opposite ends of a nesting spectrum, says John Eadie of the University of California, Davis, who coauthored a commentary on the study in the same issue of Nature. “Studying both in the same species and the same population is quite an exceptional opportunity.”
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A. Zink and J.M. Eadie. When cooperators cheat. Nature. Published online February 27, 2019. doi:10.1038/d41586-019-00643-7.