Bawling babies are the bane of many long-suffering parents. But consider the lot of a pair of birds that spends weeks building a nest, and preparing to raise a brood of their own, only to be hoodwinked into caring instead for a gluttonous interloper. That is the fate of victims of the common cuckoo, Cuculus canorus. Cuckoos are brood parasites: rather than building nests of their own, they lay their eggs in the nests of other species, relying on the unwitting foster parents to incubate the egg and feed the hatchling until it is fully fledged. But why do the hosts put up with it? Therein lies a sinister and unlikely tale, as told by Nicholas Davies and colleagues in Proceedings of the Royal Society1.
Davies et al. are interested in the battle of wits between the cuckoo and one of its host species, the reed warbler Acrocephalus scirpaceus. They studied a breeding population of about 300 reed warbler pairs in and around Wicken Fen near Cambridge in England. Reed warblers are vulnerable to cuckoos, and in some years at Wicken nearly a quarter of their nests are parasitized, although in other years only 1-2% of nests are affected. Female cuckoos surreptitiously lay a single egg in an unguarded nest. Adult reed warblers reject eggs that are unlike their own, and their powers of discrimination have been honed by natural selection. But in this coevolutionary arms race, the cuckoo has also been selected to lay eggs which beautifully mimic those of its hosts, and which are often accepted and incubated2.
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