Researchers carried out field work in the Choma district of Zambia during September to November across four years. The first step was to measure the differences in colour and pattern of the fork-tailed drongo eggs and cuckoo eggs. The team found that the colour and pattern of cuckoo eggs was, on average, almost identical to that of drongo eggs, and that all the broad types of drongo egg signatures were forged by the cuckoos.
“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,” said lead researcher and Zoology PhD student Jess Lund.
The second step involved ‘egg rejection’ experiments in which the researchers simulated cuckoo visits by ‘parasitising’ drongo nests with foreign eggs from other drongo nests (as a proxy for African cuckoo eggs). They then checked the nest daily to see whether the drongo parents accepted the foreign egg as one of their own, or realised it was an imposter and rejected it by removing it from their nest. The team could then test what differences in colour and pattern between the foreign egg and the drongo’s own eggs best predicted whether or not the drongo parents were tricked.
Co-author Collins Moya lowering an African cuckoo chick from a tree so that it can be measured before being returned to the nest. Credit: Claire Spottiswoode
Co-author Collins Moya lowering an African cuckoo chick from a tree so that it can be measured before being returned to the nest. Credit: Claire Spottiswoode
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%.
“We were surprised to see that so many of the cuckoo eggs were predicted to be rejected”, said Lund “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.”
Fork-tailed drongos have likely honed these signatures and detection abilities through natural selection, explains Lund. “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 suggest that a female cuckoo may only fledge two chicks in her lifetime – only just replacing herself and her mate. 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 researchers think that the fork-tailed drongos where the research took place could be particularly good at spotting forgeries. “Perhaps this part of Zambia is a hotspot for parasitism, where drongos have particularly fine-tuned defences, and against which cuckoos stand little chance,” said Lund.
The research was supported by the DST-NRF Centre of Excellence at the FitzPatrick Institute at the University of Cape Town, the BBSRC, the Royal Society, and the Department of Zoology at the University of Cambridge, and carried out with the crucial support of a community in Zambia who find and access drongo nests.
Reference:
Lund et al. When perfection isn’t enough: host egg signatures are an effective defence against high-fidelity African cuckoo mimicry, Proceedings of the Royal Society B, DOI: 10.1098/rspb.2023.1125
Published 26 July 2023
The text in this work is licensed under a Creative Commons Attribution 4.0 International License
