Interspecific infanticide by a Klaas’s cuckoo Chrysococcyx klaas

The coevolutionary arms race between brood parasites and their hosts has shaped a remarkable diversity of behaviours. Avian brood parasites engage in interspecific infanticide by removing or killing the offspring of potential host species, but it remains unclear how widespread or common this behaviour is, and what the underlying adaptive benefits might be. Here, we report the first documented case of nestling eviction by a Klaas’s Cuckoo Chrysococcyx klaas. In our ornithological field study, where we monitor bird nests with cameras in the savanna of eastern Eswatini, we recorded a female Klaas’s Cuckoo ejecting two five-day-old Long-billed Crombec Sylvietta rufescens nestlings from an unparasitised nest. The cuckoo did not consume the nestlings, suggesting that the behaviour was not motivated by energetic gain. Instead, since the cuckoo removed all nest contents and the event happened early in the breeding season, the infanticide may well have induced the parents to renest and thus have created future opportunities for the brood parasites (consistent with the ‘farming’ hypothesis). Alternative explanations, such as the ‘mafia’ hypothesis (whereby infanticide functions as retaliation for hosts’ rejection of brood parasites), seem less likely because the killed brood had probably not been parasitised, but cannot fully be excluded with the current information. Nonetheless, our observation adds a species to the growing list of brood parasites that engage in interspecific infanticide. While this suggests that the behaviour is adaptive and perhaps evolutionarily conserved across species, we highlight the need for targeted studies to determine the evolutionary drivers of this intriguing behaviour in this, and other brood parasitic species.