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A parasitic cuckoo can be a lifesaver: intruder chick releases putrid goop that may foil predators.

A parasitic cuckoo chick foisted upon other birds can be luck in disguise, saving the nest with a disgusting defense.

Defense by cuckoo chicks of carrion crow nests at high risk of predator attack could be the first example of a parasitic bird's benefit to its host, says Daniela Canestrari, an ecologist at the University of Oviedo in Mieres, Spain.

About 1 percent of bird species, including cuckoos, outsource childcare by leaving eggs in other birds' nests. The intruder chick often kills or outcompetes the rightful offspring.

Chicks of the great spotted cuckoo (Clamator glandarius) don't directly kill chicks of the carrion crows (Corvus corone corone), so the crow parents have a chance of producing their own offspring. When lots of predators lurk, the cuckoo chick's strong defense mechanism--releasing a caustic, stinking slime from its rear--may explain why parasitized nests are more likely to fledge a crow chick than unparasitized ones. In the right circumstances, even cuckoo parasitism can turn into a mutually beneficial relationship, Canestrari and her colleagues argue in the March 21 Science.

An example of a benefit of brood parasitism is "exceedingly unusual and cool," says Claire Spottiswoode, an evolutionary biologist at the University of Cambridge.

A previous suggestion of a brood parasite's benefit, reported nearly 50 years ago from giant cowbirds parasitizing oropendola nests in Panama, hasn't stood up to later research, says cuckoo researcher Juan Soler of the Arid Zones Experimental Station in Spain.

Supposedly the cowbird chicks would pick parasitic botflies off the rightful nestlings. Later researchers dismissed the idea, questioning whether the young cowbirds could even grasp and yank off the parasites.

In the new study, researchers analyzed 741 carrion crow nests at a site in Spain over 16 years. Nests with at least one cuckoo chick in them were more likely (76 percent versus 54 percent) to fledge at least one crow nestling than were nests with no cuckoo chicks. There was a cost, though: Among the nests that survived, those with cuckoo chicks fledged on average only 2.1 crows, fewer than the 2.6 fledglings in nests without an intruder.

To see if the benefit seen was due to cuckoos simply targeting the most robust crow nests, the team added and removed cuckoo chicks. Nests with cuckoos were still more likely to fledge at least one crow.

When disturbed by intrusions such as a researcher grabbing a nestling, spotted cuckoo chicks defend themselves--and everything else within splattering distance--by releasing dark, sticky glop that's stinkier than their usual poop. "It's terrible," says Canestrari. "The worst part is that the smell doesn't go away when you wash, but lingers for a long time."

A chemical analysis revealed what she calls "an acid bomb." The goo contains substances known to repel animals, and tests with predators such as falcons confirmed the slime's power to disgust. Only one of 12 feral cats tried even a bite of meat coated in cuckoo excretions.

"I don't think that the results are broadly applicable to brood parasites in general," says Stephen Rothstein of the University of California, Santa Barbara. Nearly all other species of parasitic cuckoos and parasitic honeyguides kill all the host young. Even with parasitic cowbirds, "which are sometimes mistakenly cited as examples of parasites that are not very harmful," he says, hosts nearly always lose all of their young because cowbirds outcompete the host chicks.

Caption: A great spotted cuckoo chick (front) doesn't directly kill the carrion crow nestlings it grows up with and, in a crisis, it may benefit the entire nest.


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Title Annotation:LIFE & EVOLUTION
Author:Milius, Susan
Publication:Science News
Geographic Code:4EUSP
Date:Apr 19, 2014
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