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Birds’ Signature Defense Thwarts Cuckoo Egg Forgeries

Fork-tailed drongos use unique egg 'signatures' to identify and reject nearly identical African cuckoo eggs.

Sneaky cuckoos are being foiled in their plot to get another bird to raise their brood by its ability to identify and reject their almost identical eggs.

African cuckoos lay their eggs in drongos’ nests to avoid rearing the chicks themselves.

Fork-tailed drongos lay eggs with a “remarkable” diversity of colors and patterns.

But all the colors and patterns are forged by the African cuckoo.

The “forged” eggs are intended to trick the drongos into thinking the cuckoo’s egg is one of their own.

Now scientists have discovered that drongos use knowledge of the color and patterns of their own personal eggs – known as the egg’s “signature” – to identify cuckoo egg ‘forgeries’ and reject them from their nests.

The research team explained that the egg “signatures” are like the signatures we use in our daily lives: unique to each individual and highly repeatable by the same individual.

Through natural selection, the African cuckoo’s eggs have evolved to look almost identical to drongo eggs – a rare example of high-fidelity mimicry in nature.

A freshly-hatched African cuckoo chick in a fork-tailed drongo nest, in the process of throwing a drongo egg out the nest. PHOTO BY CLAIRE SPOTTISWOODE/SWNS 

The research team, led by scientists from Cambridge University and the University of Cape Town in South Africa, set out to explore the effectiveness of “signatures” as a defense against highly accurate mimicry.

They found that despite near-perfect mimicry of fork-tailed drongo eggs, African cuckoo eggs still have a high probability of being rejected.

Researchers carried out fieldwork in the Choma area of Zambia from September to November across four years.

The first step was to measure the differences in color and pattern between the fork-tailed drongo eggs and cuckoo eggs.

The team found that the color and pattern of cuckoo eggs were, on average, almost identical to that of drongo eggs and that all the broad types of drongo egg “signatures” were forged by the cuckoos.

Lead researcher Jess Lund. a zoology Ph.D. student at Cambridge, said: “It is incredible how perfect the mimicry is.

“We have occasionally missed cuckoo eggs in the field because they looked exactly like the drongo clutch that they were found in.”

She said the second step involved ‘egg rejection’ experiments in which the researchers simulated cuckoo visits by ‘parasitizing’ drongo nests with foreign eggs from other drongo nests.

They then checked the nest daily to see whether the drongo parents accepted the foreign egg as one of their own or realized it was an imposter and rejected it by removing it from their nest.

The researchers could then test what differences in color and pattern between the foreign egg and the drongo’s own eggs best predicted whether or not the drongo parents were tricked.

By combining results from both steps of the study, the researchers were able to create a model that predicted how often, on average, an African cuckoo would have its eggs rejected by a fork-tailed drongo host.

They found the predicted rate of rejection to be 93.7 percent.

Lund said: “We were surprised to see that so many of the cuckoo eggs were predicted to be rejected.

“Our additional simulations show this is likely due to drongos having evolved ‘signatures’ on their eggs.

“Even though cuckoos have evolved excellent ‘forgeries’, individual cuckoos don’t target individual drongo nests that match their own eggs.

“This means that for each cuckoo egg laid, the likelihood that it will be a good enough match to that drongo’s ‘signature’ is very low.”

She says fork-tailed drongos have likely honed these “signatures” and detection abilities through natural selection.

Lund explained: “It’s very costly for drongo parents if they don’t have these skills.

“If they can’t tell a ‘forgery’ from their ‘signature’ their own chicks will be killed by the cuckoo when it hatches, and they will be stuck raising a ravenous cuckoo for a whole breeding season.”

The results of the study, published in the journal Proceedings of the Royal Society B, suggest that a female cuckoo may only fledge two chicks in her lifetime – only just replacing herself and her mate.

The researchers say that this would not amount to a sustainable population, which presents a puzzle because African cuckoos remain a common bird in many parts of Africa.

The team believes that the fork-tailed drongos where the research took place could be particularly good at spotting ‘forgeries’.

Lund added: “Perhaps this part of Zambia is a hotspot for parasitism, where drongos have particularly fine-tuned defenses, and against which cuckoos stand little chance.”

Produced in association with SWNS Talker

Edited by and

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