Question

Brown headed cowbirds lay their eggs in other birds’ nests, a behavior called nest parasitism. When this strategy succeeds, the host birds accept the cowbird egg as one of their own and rear the cowbird chick. When it fails, the host birds reject the cowbird egg as an imposter and remove it from the nest. Why do any host species accept cowbird eggs in their nests? This seems maladaptive since they are expending energy that should go toward raising their own offspring on cowbird chicks. Biologists have proposed two hypotheses: first that the parasitized species have not yet evolved the behavioral response of egg ejection yet, but eventually will. The second hypothesis is that the host birds are physically constrained since their beaks are too small to remove the large cowbird eggs from their nests.

Propose an experiment to determine which of these two hypotheses is correct.

Answer 1

An experiment can be designed in artificial settings. In a nest where the host species are living, the eggs of another parasite species which yield small sized eggs can be introduced. This is done keeping in mind the compatibility of the physical structure of the beak or the beak size of host species. This is now left and finally observed for the fact if the foreign eggs have been removed from the nest of that parasitized species.

Case1: If we observe that the small sized foreign eggs of the parasite species are removed by the host, then we can conclude that it is the physical constraint which inhibits the host to remove the parasite's egg. Since the reduction of the size of the egg facilitates its clearance by the host, hence small size is compatible to be removed by the beak.

Case 2: If we observe that the small sized egg of the parasite is still not removed from the nest of the host, then it is not the physical constraint of the beak size but rather the deficit of the behavioural evolution of the host species. This behavioural change involving egg ejection may occur in the future.