Asian Koels do not lay eggs matching the host eggs' phenotype

Whether cuckoos randomly choose host nests in which to lay their eggs, or strategically choose host nests containing eggs that best match their own in appearance, is not well understood. In this study, combining avian visual modelling and a controlled field experiment, we objectively studied the ‘egg‐matching hypothesis’ using the brood parasitic Asian Koel Eudynamys scolopaceus and one of its main hosts in Bangladesh, the Common Myna Acridotheres tristis . Common Mynas rarely reject foreign eggs, so we expected that Asian Koels would not differentiate between egg appearances. To directly assess nest selection behaviour, we installed 100 paired nestboxes – each pair comprising one nest with restricted koel access and another accessible nest. We demonstrate that Asian Koels do not differentiate between active Common Myna nests or dummy nests with Common Quail Coturnix coturnix eggs or dummy nests without quail eggs during egg‐laying. Parasitism was also significantly less frequent (2.1%, n = 141) in boxes with smaller openings that were less accessible to koels, whereas 27.8% ( n = 18) of myna nests in nestboxes with large openings were parasitized. Koels parasitized 23.4% ( n = 141) of dummy nests with eggs in nestboxes with large openings. None of the dummy nests in boxes with small openings were parasitized. Koels laid eggs in active myna nests or dummy nests only when at least one active myna clutch was present. This lack of discrimination between these different experimental groups suggests that Asian Koels were not matching host eggs in this experiment. In addition, Common Myna eggs from parasitized nests did not appear more like Asian Koel eggs compared with Common Myna eggs from unparasitized nests. Together, these findings suggest that, rather than attempting to lay eggs in nests that best match their own eggs in appearance, Asian Koels lay eggs that poorly match in colour to the local hosts' eggs.