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To fool other birds, cuckoos use multiple disguises

Cuckoo chick in a reed warbler nest at Wicken Fen. [Image by N.B. Davies]

Social learning, or learning from the behavior of others, might play a larger role in animal mimicry—or the art of disguise—than researchers ever realized, according to a new study of common cuckoos.

Cuckoos are known as brood parasites, which means that they sneak their eggs into other birds' nests so that they don't have to raise their own chicks. Some female cuckoos have evolved feathers that resemble a sparrow hawk, a common predator of song birds. These disguised, or mimicking, cuckoos are able to approach the nests of other birds, like reed warblers, without being attacked by the nests' owners.

The mimicking cuckoos come in two different forms—either gray or reddish-brown—and a new study by Rose Thorogood and Nicholas Davies shows that, whenever reed warblers become brave enough to attack intruding cuckoos, other reed warblers in the area take notice. According to the researchers, reed warblers that witness other warblers attacking a cuckoo are more likely to defend their own nests against mimicking cuckoos of the same color.

However, cuckoos of the other color are still able to fool the reed warblers and sneak into their nests without being attacked, they say.

Based on these results, Thorogood and Davies suggest that the social information spread among reed warblers—about which mimicking cuckoos to attack—has led to the evolution of both gray and reddish-brown color in the cuckoos. So, even when reed warblers have learned to attack one colored cuckoo, the other colored cuckoo remains a successful brood parasite.