Fork-tailed Drongos can detect and reject cuckoo eggs from their nest despite them on average looking almost identical to drongo eggs, according to new research.
The drongo lays eggs with an astounding diversity of colours and patterns – all of which are 'forged' by African Cuckoo. However, drongos use knowledge of their own personal egg 'signatures' to identify cuckoo egg 'forgeries' and reject them from their nests.These signatures are the colours and patterns of the eggs, which are both unique to and highly repeatable by the same individual Fork-tailed Drongo.
Fork-tailed Drongos use knowledge of their egg colours and patterns to identify and reject African Cuckoo eggs (Dave Williams).
A team led by researchers at the University of Cambridge and the University of Cape Town, working in collaboration with a community in Zambia, set out to explore the effectiveness of signatures as a defence 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.
"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.
Following two studies, 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. That figure was as high as 93.7% of the time.
"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."
Reference
Attwood, M C, Dixit, T, Hamama, S, Jamie, G A, Lund, J, Moya, C, Spottiswoode, C N, & Stevens, M. 2023. When perfection isn't enough: host egg signatures are an effective defence against high-fidelity African cuckoo mimicry. Proceedings of the Royal Society B: Biological Sciences. DOI: https://doi.org/10.1098/rspb.2023.1125
