Popular Science: Cuckoos’ Russian Roulette
Interspecific brood parasitism is one of the successful reproductive strategies in birds. In the course of evolution, both the parasites and the hosts have developed many adaptations to help them win over the other. Parasitic females shortened the time of laying eggs in the host nests, improved the timing and the egg mimicry, which increased resemblance of the host eggs in size and colour.
There is one feature of the process that we still do not understand, and that is the removal of host egg during the act of parasitism. Most of the parasitic bird species do so, including the common cuckoo (Cuculus canorus). Michal Šulc from our Department of Ecology together with colleagues from the Academy of Sciences focused on two hypotheses that may explain this behaviour. Both examine if the cuckoo female removes a specific egg to improve the chances of her offspring to survive and reproduce. The first hypothesis says that the cuckoo removes the egg of another cuckoo laid to the nest earlier. The second one is newer and proposes that the cuckoo female chooses the egg least similar to her own to increase its mimicry in the host clutch.
The scientists were observing the nests of great reed warblers in South Moravia, who are the favourite target of common cuckoos. To examine the first hypothesis, they observed the presence of eggs laid by different cuckoo females. The second one was tested by comparing the colour and spottiness of eggs.
Neither of the hypotheses was corroborated, and it seems that the cuckoo simply removes one of the original eggs completely randomly. It is possible that the cuckoos are under time pressure. The great reed warblers can be fairly aggressive when catching a cuckoo in their nest, and they are even able to kill her. They also refuse to take care of the alien egg more often. Therefore spending more time in the nest by choosing the right egg to remove might be potentially fatal. Also, the host female lays eggs even after the cuckoo visit, and therefore might lay the least similar ones only after the cuckoo leaves.
Future studies should therefore focus more on different hypotheses. For now, the evolutionary purpose of cuckoo egg removal remains a scientific mystery.