Cuckoo Alert: Warblers Set Up 'Neighborhood Watch' to Protect From Parasitic Birds

During the breeding season, reed warblers set up a sort of "neighborhood watch," Cambridge University researchers reveal in a new study. This line of defense, they say, is to protect warbler nests from popular parasitic birds, the cuckoos.

Cuckoos are expert tricksters, able to produce eggs of varying color to mimic those of their local hosts'-reed warblers, for example. To best disguise their eggs, cuckoos lay only one egg per nest in a matter of ten seconds - ensuring they aren't spotted by the parents.

However, when a cuckoo chick hatches, it pushes the reed warbler eggs and young out of the nest and tricks warbler 'parents' into raising it to full-fledgling. (Scroll to read more...)

In the latest study, researcher from the University of Cambridge examined the interactions among several neighboring communities of reed warblers and the strategies they used for coping with cuckoos. Using a series of controlled experiments, involving model cuckoos and broadcasts of reed warbler alarm calls, researchers found warblers actually gather information from a variety of local nests, keeping birds up-to-date with the latest threats, according to a news release.

When they aren't doing all that, warblers embark on an annual migration of some 5,000 kilometers, from their West African winter homes to breeding grounds in the north each May.  Although it is often difficult for a warbler to distinguish between its own egg and a parasitic cuckoo's, researchers revealed warblers paid close attention to those eggs laid in neighbor nests, and factored this information into their decision on whether or not to eject an egg from their nests..

For example, when reed warblers spot a cuckoo, they may mob it and emit alarm calls that carry up to 40 meters, attracting neighbors to the scene. However, this is insufficient cause for warblers to eject a suspect egg from their own nests.

"We found that warbler pairs ejected an odd egg only when there was strong evidence that it might not be one of their own. For action to be taken, the clues had to add up: the warblers needed to be alerted by their neighbors' behavior that there was a cuckoo at large in the neighborhood and they needed to be aware of a more local and imminent threat, by seeing a cuckoo near their own nest," Rose Thorogood, co-author of the study from Cambridge's Department of Zoology, explained in the release.

In turn, this makes it more difficult for cuckoos to fly under the radar, as their neighborhood is abuzz with information regarding their whereabouts.

"Because the information warfare between cuckoos and their hosts extends well beyond individual interactions, there's pressure on cuckoos to be increasingly secretive, not only to avoid alerting their target host pair, but also other host pairs in the local neighborhood," Thorogood added.

Consequently, cuckoo populations have declined by as much as 60 percent in the last 30 years. Generally between 10 and 20 percent of warbler nests are used by cuckoos. Recently, hwoever, researchers found only two percent of warbler nests host cuckoos.

"Reed warblers are much less likely to eject an egg from their nest today than they were in the 1980s. This makes complete sense. They have matched their behavior to the changing level of risk," co-author Nicholas Davies concluded in the university's release. "Most reed warblers have just one or two summers in which to breed. So every opportunity to mate, construct a nest and raise a clutch of eggs is precious.

"If a pair of warblers mistakenly identifies one of their own eggs as a cuckoo egg and chucks it out, or deserts the nest, the loss is great," he added.  "Our work shows how they match their defenses to the risk of parasitism."

Their study was recently published in the journal Scientific Reports. 

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