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Looking like a hawk stops birds mobbing cuckoos

31 March 2011, by Tamera Jones

Scientists have found that as well as craftily laying their eggs in other birds' nests, cuckoos have another trick up their sleeves that helps them take advantage of other birds.

Reed warbler feeding a cuckoo

Reed warbler feeding a common cuckoo chick in a nest

If a cuckoo's plumage looks like a hawk's, hosts like the reed warbler are less likely to attack them as they try to lay eggs in their nests.

Cuckoos are so-called brood parasites, because they lay their eggs in other birds' nests so they don't have to go to the trouble of raising their own chicks. A newly hatched cuckoo chick then commits a shocking act: it hoists the hosts' eggs or newly hatched chicks onto its back and turfs them out of the nest.

But how does the cuckoo manage to lay its eggs in reed warblers' nests in the first place?

'Reed warblers are afraid of sparrowhawks, which are lethal birds of prey, and cuckoos exploit this by mimicking the sparrowhawk's barred underparts,' says Dr Justin Welbergen from the University of Cambridge, lead author of the study, published in Behavioral Ecology.

'So if parent reed warblers mistakenly think a sparrowhawk is near their nest, they are going to keep out of the way out of fear for their lives,' he explains.

The intruding cuckoo is then free to lay her egg without being mobbed by the hosts.

'Ironically, having a cuckoo lay its egg in your nest is very costly and compares to being attacked by a hawk,' says Welbergen.

'Reed warblers are afraid of sparrowhawks, which are lethal birds of prey, and cuckoos exploit this by mimicking the sparrowhawk's barred underparts.'
Dr Justin Welbergen, University of Cambridge

This is the first time researchers have shown that having hawk-like barred plumage on their bellies benefits cuckoos. Welbergen and co-author Professor Nick Davies – also from the University of Cambridge – say the cuckoo has evolved a hawk-like appearance to stay one step ahead of its hosts.

The findings are a brilliant example of a so-called coevolutionary arms race, where two parties evolve ever more elaborate tricks to outwit each other.

Many people – including Aristotle – have noted since ancient times how similar cuckoos are to hawks. Indeed earlier research by Davies and Welbergen had revealed that blue tits and great tits, which have no experience of brood parasitism, are just as afraid of common cuckoos as they are of sparrowhawks.

But before this latest study, no-one had looked to see if the birds that cuckoos actually parasitize - like reed warblers – also treat cuckoos as hawk-like.

So, Welbergen and Davies decided to show wild reed warblers taxidermic models of sparrowhawks, cuckoos, and collared doves at 36 reed warbler nests at Wicken Fen, Cambridgeshire, UK. Sparrowhawks and cuckoos both have barred underbellies, but collared doves have plain underparts. So the doves served as a control.

To test what difference the barred plumage makes, the researchers pinned pieces of white silk on the bellies of the cuckoo and sparrowhawk models to hide the barred bellies, then recorded the sounds the birds made and watched to see if they mobbed.

They found that reed warblers are more reluctant to mob cuckoos with barred rather than unbarred bellies. This means that their mimicry of hawks helps cuckoos to gain better access to reed warblers' nests to lay their parasitic eggs.

'Because the parasite trickery is costly to the warblers they are likely to evolve the ability to discriminate cuckoos from hawks, which in turn, should favour the evolution of improved hawk mimicry in cuckoos. This is analogous to earlier findings that show that where hosts have evolved egg or chick discrimination, brood parasites have evolved mimicry of host eggs or chicks,' says Welbergen.

'Our study identifies mimicry as a general brood parasitic strategy for circumventing host defences, and highlights coevolution between mimicry and discrimination as a common feature of the evolutionary arms races between brood parasites and their hosts,' he adds.

Interestingly, in Asia there are related brood parasites that look even more hawk-like than the Common cuckoo. 'These higher degrees of hawk resemblance are expected to be accompanied by higher degrees of host discrimination,' write the authors in their study.

Justin A. Welbergen and Nicholas B. Davies, A parasite in wolf's clothing: hawk mimicry reduces mobbing of cuckoos by hosts, Behavioral Ecology, First published online: March 21, 2011, doi: 10.1093/beheco/arr008

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