This Freeloading Bird Brings Help — And The Help Smells Gross

The great spotted cuckoo is a parasitic bird that plops its eggs in nests of other birds, so others can care for its chicks. Those chicks might aid the caretaker bird by helping to repel predators.

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The great spotted cuckoo is a sneaky bird. It lays its eggs in the nests of other species such as crows. Baby cuckoos grow up side by side with the crows that belong in that nest, gobbling up food and the parents' attention. You'd think that this would be a bad thing for those crows. But as NPR's Nell Greenfieldboyce reports, this kind of freeloading can sometimes be helpful.

NELL GREENFIELDBOYCE, BYLINE: Daniela Canestrari has studied carrion crows for years. She works at the University of Oviedo in Spain. She says, often, baby crows just disappear from the nest because they've become some animals' lunch. But recently, she and her colleagues noticed something weird. Crow nests that also contained cuckoos were more successful. They were more likely to produce at least one crow fledgling than the cuckoo-free nests.

DANIELA CANESTRARI: Well, when we analyzed the data and we found this pattern, we were really, really puzzled and amazed.

GREENFIELDBOYCE: Because after all, weren't cuckoos harmful parasites? It didn't make sense.

CANESTRARI: And so we started to think about possible explanations.

GREENFIELDBOYCE: One explanation was that adult cuckoos might have some way of knowing which crow nest would be the most successful. Maybe they selected those nests for their eggs. So the researchers did an experiment. They took cuckoos out of some crow nests and put them into other crow nests. What they found is that it was the cuckoo chick that seemed to be helping the crows.

CANESTRARI: The nests where we removed a cuckoo chick were more likely to fail. And the nests where we added a cuckoo chick were more likely to be successful.

GREENFIELDBOYCE: The question was, why? The researchers had noticed something that might be the answer, something gross. Whenever they touched a baby cuckoo in the nest, it excreted something that wasn't feces. It was different.

CANESTRARI: Well, it's kind of blackish, it's liquid, sticky, and it has kind of rotten smell. And it is something I couldn't describe because it doesn't resemble anything I've smelled in my life, but it's really, really bad.

GREENFIELDBOYCE: So bad that it might protect the nest by repelling predators. Her team tested this idea by putting the noxious substance into chicken meat and then trying to feed it to feral cats. In the journal Science, they report that the cats wouldn't touch the stuff. The chemical analysis showed that it contains all kinds of caustic compounds.

SCOTT ROBINSON: This is really a fascinating study. I mean, I really, really wonder where those cuckoo nestlings are getting all those toxic chemicals.

GREENFIELDBOYCE: Scott Robinson is a bird ecologist at the Florida Museum of Natural History in Gainesville.

ROBINSON: What this study shows is that there is a potential advantage to being parasitized, something that was proposed a great many years ago but never replicated.

GREENFIELDBOYCE: He says this is a great example of how in nature, what looks like exploitation might sometimes have hidden benefits. Nell Greenfieldboyce, NPR News.

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