Outwitting deadbeat cuckoo finch parents

Some bird species in Zambia have evolved novel strategies to evict eggs left by parasitic cuckoo finches.

African cuckoo finches just aren’t into parenting. They lay their eggs in the nests of other birds, leaving those birds with the hard work of raising their cuckoo finch chicks. But, according to recent research by Dr. Claire Spottiswoode of the University of Cambridge’s Department of Zoology, some bird species are getting wiser to it, having evolved ways to identify and remove cuckoo finch eggs from their nests. Her results were published in the April 13, 2011 issue of Proceedings of the Royal Society B (Biological Sciences).

Cuckoo finch. Photo taken in Midmar Game Reserve in South Africa. Image Credit: Alan Manson, via Wikimedia Commons.

Birds that abandon their parenting duties by leaving their eggs in nests for other birds to raise are known as brood parasitic birds. The female cuckoo finch, for instance, lays eggs that resemble those of the unwitting “host” birds, to trick them into raising her chick.

The tawny-flanked prinia has found an unusual way to outwit the cuckoo finches, by laying eggs with different colors and markings. Those unique egg signatures help the prinias distinguish between their eggs and those from the cuckoo finch since the color and pattern of eggs from the female cuckoo finch remains the same over her lifetime. Because she can’t keep up with the egg diversity of the tawny-flanked prinia, her eggs are usually noticed and ejected from the nest. A movie clip, below, courtesy of Dr. Claire Spottiswoode, shows a prinia removing a cuckoo finch egg from her nest.

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Tawny-flanked prinia. Image Credit: Alan Manson, via Wikimedia Commons.

Said Dr. Spottiswoode in a press release,

As the cuckoo finch has become more proficient at tricking its hosts with better mimicry, hosts have evolved more and more sophisticated ways to fight back. Our field experiments in Zambia show that this biological arms race has escalated in strikingly different ways in different species. Some host species – such as the tawny-flanked prinia – have evolved defenses by shifting their own egg appearance away from that of their parasite. And we see evidence of this in the evolution of an amazing diversity of prinia egg colors and patterns.

These variations seem to act like the complicated markings on a banknote: complex colors and patterns act to make host eggs more difficult to forge by the parasite, just as watermarks act to make banknotes more difficult to forge by counterfeiters.

Eggs of another cuckoo finch host, the red-faced cisticola, are not as diversely patterned and colored as those of the tawny-flanked prinia. But the cisticolas have become “smarter” by developing a more sophisticated way to distinguish the differences between their eggs and those of the cuckoo finches.

Red-faced cisticola, non-breeding adult. Image Credit: Alan Manson, via Wikimedia Commons.

Dr. Martin Stevens, the paper co-author, also at the Department of Zoology at the University of Cambridge, commented on these findings in the same press release:

Our experiments have shown that these different strategies are equally successful as defenses against the cuckoo finch. Moreover, one species that has done a bit of both – the rattling cisticola – appears to have beaten the cuckoo finch with this dual strategy, since it is no longer parasitized. The arms race between the cuckoo finch and its host emphasizes how interactions between species can be remarkably sophisticated, especially in tropical regions such as Africa, giving us beautiful examples of evolution and adaptation.

A comparison of cuckoo finch eggs and their hosts’ eggs. Image Credit: Dr. Claire Spottiswoode.

The cuckoo finch, a brood parasitic bird that lays her eggs in the nests of other birds, leaving them to raise her chicks, is out of luck when it comes to at least three potential hosts. The tawny-flanked prinia, red-faced cisticola, and rattling cisticola have evolved ways to outsmart her, either by laying a diverse variety of eggs that she cannot mimic or become more discriminating in identifying her eggs, or both. It’s a remarkable example of natural selection at work in the evolutionary competition for survival.

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