Asian koels do not lay eggs matching the host eggs’ phenotype

Fieldwork was conducted on the Jahangirnagar University campus (23˚52´ N, 90˚16´ E), Bangladesh. The study area is a heterogenous landscape (approximately 280 ha) and includes woodlands, grasslands, wetlands, cultivated lands and human settlements (Nahid et al., 2020). Multiple cuckoo species differing in size from small to large coexist and breed sympatrically in the study area (Nahid et al., 2016a, b).Study speciesThe koel is a generalist, non-evictor brood parasite distributed throughout Asia (Ali and Ripley, 1981; Payne, 2005; Erritzøe et al., 2012). It is the most common resident among the cuckoos in the study area. A total of 16 passerine species from different areas have been recorded as hosts of the koel (Mann, 2017). In our study area, the koel regularly parasitizes nests of house crows (Corvus splendens), mynas and long-tailed shrikes (Lanius schach) (Begum et al., 2011; Nahid et al., 2020, 2021). The koel clutch size is usually 2-3, but up to 13 koel eggs have been recorded in one host nest (Payne, 2005; Erritzøe et al., 2012; Nahid et al., 2021).The myna is widely distributed in south and south-east Asia and is one of the most exploited hosts by the koel in our study area with the parasitism rate between 31.2% and 33.6% (BirdLife International, 2017; Craig et al., 2019; Nahid et al., 2021). Additionally, a recent study found that 21.2% (n=99 nests) of mynas that nested in nest boxes were parasitised in our study area (Nahid et al., 2024). The myna builds nests most often in cavities, close to human habitation, as well as in natural areas (Lamba, 1963; Ali and Ripley, 1981; Begum et al., 2011b). It breeds from March to August in Bangladesh, but it can breed all through the year depending on the area (Begum et al., 2011a, Nahid et al., 2020). The clutch size is usually 4-5 eggs (Nahid et al., 2021).In this study, we demonstrate that Asian koels lay randomly, and do not differentiate between active common myna nests or dummy nests with domestic quail (Coturnix coturnix) eggs or dummy nests without quail eggs. This lack of discrimination between these different experimental groups, strongly suggests that Asian koels were not matching host eggs in this experiment. In addition, common myna eggs from parasitized nests did not appear more similar to Asian koel eggs compared to common myna eggs from unparasitized nests. Together, these findings present convincing results showing that, rather than attempting to lay eggs in nests that best match their own in appearance, Asian koels lay eggs indiscriminately in any potential active common myna nest.