Natural Imposters? A Cuckoo View of Social Relations

Perched on a branch, a cuckoo hen watches in silence, waiting for a robin to leave her nest. Once the robin leaves to forage, the cuckoo approaches its nest, and crouches amid the robin’s eggs. Within seconds the cuckoo lays its egg and flies off. In colour the cuckoo’s egg approximates that of the robin. The robin soon returns to resume brooding her clutch. As far as we can tell, she does not react to the appearance of an additional egg now in her nest. On hatching, the robin raises the cuckoo nestling alongside her biological offspring. The cuckoo chick outcompetes some of the nestling robins. This results in the robin chicks’ death by starvation, and the ejection of their corpses from the nest. Upon sexual maturity, the surviving offspring – cuckoo and robins alike – go on to repeat this ecological cycle.

Ecologists describe this behaviour as brood parasitism. Cuckoos are a diverse family of 141 species ranging across all continents bar Antarctica. Not all cuckoo species are brood parasites. Those that are lay their eggs in the nests of other birds. Their eggs typically exhibit morphological features resembling those of the host, as we described with the robin. Seemingly none the wiser, the host birds rear the cuckoos’ offspring, often to the demise of their biological offspring. By mimicking host species egg morphology (and in some cases consuming the host bird’s egg; Fromme, 2018) the cuckoo reproduces without engaging in parental care. In this way, cuckoo behaviour conjures the idea of imposture in the most intimate of contexts.

In the opening vignette, we tried to depict the cuckoo’s actions in purely descriptive terms.