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Cracked! The two million-year-old egg forgery scandal solved by Cambridge researchers this Easter



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An egg forgery scandal two million years in the making has been cracked in time for Easter.

Genetic research has answered questions puzzling scientists about ‘brood parasitism’ - in which some birds lay their eggs in the nest of other species, enabling them to avoid the challenges of parenthood.

Cuckoo finch and host chicks. Picture: University of Cambridge
Cuckoo finch and host chicks. Picture: University of Cambridge

For a century, scientists have wondered how a single brood-parasitic bird species can simultaneously mimic the eggs of several different bird species to trick them into raising their young. And it has not been known how these parasitic forgers pass on this ability to their young, despite interbreeding between birds raised by different hosts.

Prof Claire Spottiswoode from the University of Cambridge’s Department of Zoology and the FitzPatrick Institute of African Ornithology, University of Cape Town, and Prof Michael Sorenson at Boston University, explored egg mimicry in the cuckoo finch, which exploits many warbler species in Africa.

Cuckoo finch egg in zitting cisticola nest. Picture: University of Cambridge
Cuckoo finch egg in zitting cisticola nest. Picture: University of Cambridge

Their study, published in PNAS, validates a theory first proposed in 1933 by showing that female cuckoo finches inherit their ability to mimic the appearance of their hosts’ eggs from their mothers, via the female-specific W chromosome. The maternal inheritances means they avoid the risk of inheriting the wrong mimicry genes from a father raised by a different host.

And this has allowed distinct lineages of cuckoo finch females to evolve specialised egg mimicry of several different host species.

But Dr Spottiswoode said: “In this particular coevolutionary arms race between species, natural selection has created a double-edged sword.

A tawny-flanked prinia with field assistant Tom Hamusikili in Zambia. Picture: University of Cambridge
A tawny-flanked prinia with field assistant Tom Hamusikili in Zambia. Picture: University of Cambridge

“While maternal inheritance has allowed cuckoo finches to exploit multiple host species, it’s likely to slow their ability to evolve counter-adaptations as their hosts evolve new defences. In particular, parasites face a daunting challenge because some host species have in return evolved an astonishing diversity of egg colour and pattern ‘signatures’, that help hosts to distinguish their own eggs from parasitic mimics.”

If host parents do not detect and remove the parasitic egg, the young cuckoo finch typically outcompetes the hosts’ own hatchlings, which soon starve to death.

Collins Moya and Kiverness Moono conducting fieldwork. Picture: University of Cambridge
Collins Moya and Kiverness Moono conducting fieldwork. Picture: University of Cambridge

But grass-warblers have become skilled at rejecting eggs that differ from their own in colour and pattern, and all four species have evolved the ability to deposit unique ‘signatures’ onto their own eggs.

Cuckoo finches have responded by evolving mimicry of the eggs of their host species, but can also mimic at least some of the signature-like variation seen in the eggs of different females within each host species

But cuckoo finches face a challenge as they cannot recombine the different forgery traits evolved by their separate family lines.

Cuckoo finch eggs laid by different females. Picture: University of Cambridge
Cuckoo finch eggs laid by different females. Picture: University of Cambridge

Prof Spottiswoode said: “Cuckoo finches are missing out on a powerful source of evolutionary novelty and that could prove costly in this ongoing arms race. The way they inherit their ability to mimic host eggs has a downside by likely making the grass-warblers’ defences more effective, and constraining the parasite’s ability to respond.

“We may see the emergence of unforgeable egg signatures which could force cuckoo finches to switch to other naïve host species. Or the parasitic birds might become increasingly dependent on young host individuals that haven’t yet learned their own signatures and are bad at spotting mismatched eggs.”

Cuckoo finch habitat at Zambian study site. Picture: University of Cambridge
Cuckoo finch habitat at Zambian study site. Picture: University of Cambridge

The researchers believe “selection from host defences drove cuckoo finches to transfer control of egg appearance to the maternally inherited part of the genome” at least 2 million years ago.

The team collected DNA samples from 196 cuckoo finches from 141 nests belonging to the four grass-warbler species. They studied the majority by sequencing thousands of short segments across their genomes.

Cuckoo finch mimicry of tawny-flanked prinia eggs. Picture: University of Cambridge
Cuckoo finch mimicry of tawny-flanked prinia eggs. Picture: University of Cambridge

Visit africancuckoos.com to learn more.

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