Cuckoo finch fools host with multiple eggs

Cuckoo finch chick in nest Cuckoo finch chicks can go undetected in prinia nests

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Cuckoo finches lay multiple eggs in the nests of other birds to make it harder to detect the 'imposters', researchers have found.

The name cuckoo is common to many parasitic bird species that rely upon others to raise their young.

African tawny-flanked prinias are the targets of cuckoo finches in southern Zambia.

Prinias' egg colours vary widely but by laying multiple eggs in a nest, cuckoo finches reduce the risk of rejection.

The results of the study undertaken by researchers from the University of Exeter and the University of Cambridge are published in the journal Nature Communications.

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"Many brood parasites [such as cuckoo finches] and hosts are locked in ongoing evolutionary arms races, with parasites evolving attack strategies to get their eggs accepted - such as egg mimicry - and hosts evolving defences - such as egg rejection," explained co-author Dr Martin Stevens from the University of Exeter.

"Our work shows that the cuckoo finch has evolved another novel strategy of attack, whereby it defeats both sensory and cognitive components of host rejection behaviour."

The female African tawny-flanked prinia, a species of warbler, lays eggs in a wide spectrum of colours in an effort to fool cuckoo finches.

Although cuckoos have adapted their egg patterns to mimic those of their hosts they cannot guarantee their eggs will exactly match those of their chosen nest.

"Having highly variable eggs among individuals makes it hard for each female cuckoo finch to match many of the egg types that different prinias lay," explained Dr Stevens.

To understand more about prinias' egg rejection behaviour, the scientists experimented by mixing the clutches in birds' nests.

Prinia eggs and cuckoo eggs Cuckoo finches' eggs (right) can closely mimic those of African tawny-flanked prinias (left)

One theory was that the birds reject eggs based on their appearance compared with the rest of the clutch.

Alternatively, researchers thought the birds could be using a learnt "internal template" of what their eggs should look like to identify imposters.

Results of the trials suggested the birds combined the two methods to asses the colour and pattern of the eggs.

But as a result of comparing the eggs with others in the clutch, the birds struggled to successfully identify and reject the parasites as the proportion of foreign eggs in the nest increased.

Researchers found that the cuckoo finches repeatedly targeted the same nests to improve the probability of their eggs being cared for.

"In general there are many species of brood parasite that we don't know much about and it will be exciting in the future to see what strategies they have to successfully parasitise hosts," said Dr Stevens.

"It would be great to know whether other parasites have a similar strategy to the cuckoo finch, and whether there is any way hosts can fight back."

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