The African cuckoo (Cuculus gularis) is a true master of forgery. Through natural selection, its eggs have evolved to be almost perfect mimics of the eggs of the Mourning Drongo (Dicrurus adsimilis). The benefit: the cuckoo can lay its eggs in the nests of the drongos and does not have to raise its chicks itself. In this behavior, known as brood parasitism, the cuckoo chick even kills the drongo offspring after hatching – provided the cheating is not noticed!
Mourning drongos live in sub-Saharan Africa and lay eggs of a wide variety of colors and patterns. Some examples can be seen in the picture. The African cuckoo is able to imitate all these different appearances with deceptive realism. Nevertheless, scientists from the University of Cambridge and the University of Cape Town have now observed that the cuckoo eggs are rarely recognized by the drongos. According to their surveys, the drongos manage to identify foreign eggs and remove them from their nests with a probability of 93.7 percent. But how do they track down the intruder?
The solution lies in the personal “signature” of the Drongo eggs. They have specific colors and patterns that are unique to each individual and can be easily reproduced by them. In this way, drongo pairs can very effectively distinguish their own eggs from those of other bird species, but also from eggs of other drongos.
To understand what differences in egg appearance are enough for the drongo to unmask the cuckoo eggs, researchers conducted four years of experiments on drongo nests in Zambia. Instead of African cuckoo eggs, they even placed eggs from other Drongo pairs in the clutches. The team found that certain anomalies are particularly well recognized by the Drongos.
Although the African cuckoo can mimic all typical signature species, individual cuckoos do not always lay their eggs in the individual drongo clutches that match their own eggs. “Therefore, the probability that the cuckoo’s egg fits the signature of this one drongo well enough is very small,” says lead author Jess Lund from the University of Cambridge. “However, this part of Zambia could be a hotspot of parasitism, where the drongos have a particularly finely tuned defensive behavior that cuckoos have little chance against.”