To obtain better access to warbler nests, cuckoos mimic raptors

The first sparrowhawk of spring

Mimicry - The first sparrowhawk of spring | Science & technology | The Economist

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THE resemblance between cuckoos and sparrowhawks has been a cause of comment since ancient times. As the photographs above show, the two birds (cuckoo on the left, sparrowhawk on the right) are similar in size, shape and plumage, and both have a conspicuous pattern of bars on their breasts—a pattern found on many species of hawk, but not on the cuckoo's relatives. Until now, though, no one has thought to take that observation any further, and test whether the one is truly mimicking the other. Cuckoos would have good reason to do so, since the warblers and other small birds in whose nests they lay their eggs are understandably frightened of hawks, and give them a wide berth. Such leeway would make it easier for a female cuckoo to lay where she chose.

Nick Davies and Justin Welbergen, a pair of ornithologists at Cambridge University, have now corrected that omission. They have tested the idea that cuckoos mimic hawks—or, at least, that warblers think cuckoos look hawklike. As they report in the latest edition of Behavioral Ecology, they have found that it is true, but only up to a point.

Nick Davies and Justin Welbergen, a pair of ornithologists at Cambridge University, have now corrected that omission. They have tested the idea that cuckoos mimic hawks—or, at least, that warblers think cuckoos look hawklike. As they report in the latest edition of Behavioral Ecology, they have found that it is true, but only up to a point.

To carry out their test, Dr Davies and Dr Welbergen put stuffed birds near reed-warbler nests, to observe the nestholders' reactions. They used three species: sparrowhawks (which are a direct threat to a warbler's life), cuckoos (which do not harm adult warblers, but do stop them reproducing successfully) and doves (included as a control, since they pose no threat at all). The breast of each stuffed bird was adorned with a piece of white silk. Some pieces of silk were left plain. Others had bars drawn on them with a felt-tip pen. Once the stuffed birds were in place, the two researchers recorded the reactions of the nestholders. In particular, they measured how closely the nestholders were willing to approach the stuffed intruder, and also how noisily the intruder was mobbed, in an attempt to drive it away.

The barring, they found, was crucial. Though the warblers could clearly tell the difference between the species (they mobbed the cuckoos most and stayed farthest away from the sparrowhawks), in the cases of all three types of stuffed bird those decorated with stripes were approached more cautiously and mobbed less than those without stripes.

Hawk-like stripes do, then, provoke what is, from a cuckoo's point of view, the desired reaction: less of the sort of behaviour that is likely to interfere with her egg-laying mission. Not perfect mimicry, but good enough to be useful. Probably, Dr Davies and Dr Welbergen have caught cuckoo and warbler in the middle of an evolutionary arms race. Come back in 10,000 years and cuckoos and sparrowhawks may be indistinguishable.

Mimicry

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The first sparrowhawk of spring

To obtain better access to warbler nests, cuckoos mimic raptors

Science & technology April 9th 2011

From the April 9th 2011 edition

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