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BIOCEV

Why does the cuckoo win the battle with its host?

New knowledge from biologists of the ASCR
The Common Cuckoo is an obligatory brood parasite that lays its eggs in the nests of other bird species (hosts), and leaves them with the entire burden of raising their offspring. As a consequence of this way of life, a number of specific adaptations have arisen, to which the hosts respond with adequate counteradaptations. That leads to the emergence of a coevolutionary struggle between the two involved parties. One of the most interesting adaptations of the Common Cuckoo is the often noteworthy similarity of their eggs to the hosts’ eggs (mimicry), which increases the chance that the host does not recognise and remove them.

 

Each female cuckoo lays eggs of a single type, which more or less resembles the eggs of the host species on which the given female specialises, but had not yet been known whether the cuckoo female selects also a specific nest within the host population. This question waited for its solution several centuries. Now, a group of scientists in Brno from the Institute of Vertebrate Biology of the ASCR, v. v. i., has brought a possible explanation, which they have published in the prestigious scientific journal Proceedings of the Royal Society B.


Because birds, unlike humans, perceive also the ultraviolet part of the light spectrum, it was first necessary to measure the reflection waves of tens of eggs of cuckoos and clutches of Eurasian Reed Warblers, which are their common hosts in South Moravia, with a portable spectrophotometer. The data acquired in this way then converted by the authors using a mathematical model, so they could look at the whole problem from the perspective of the bird’s eye. In the end, they came to a series of statistical analyses, whose conclusion was very surprising. The eggs of the cuckoos in the naturally parasitized nests were more similar to the host eggs (in terms of colour) than the clutches randomly selected by a simulation. The authors drew the same conclusions when they compared the mimicry of the cuckoo eggs in naturally parasitized nests with the closest non-parasitized nests.


These results convincingly prove that the cuckoo is able to select the host nests actively, which lowers the likelihood of the recognition of its eggs by the host. It proves that the cuckoo can through its behaviour create selection pressure on the host without having to improve the mimicry of its eggs.


Photo: Cuckoo eggs (upper right) were successfully put in the nest of a Eurasian Reed Warbler.

 

Prepared by: Institute if Vertebrate Biology of the ASCR
 

25 Nov 2011