Honeyguides are brood parasites, cheats of the bird world that, like cuckoos, exploit the parental care of other birds to raise their young. The young birds hatch from the egg equipped with a pair of needle-sharp hooks at the tips of their beaks, and the researchers’ graphic video evidence reveals that they repeatedly grasp, bite and shake chicks of their host family until they eventually die.
"The killing behaviour is actually the culmination of a sequence of specialised adaptations that ensure that the young honeyguide has sole access to the food the host parents bring to the nest," said lead author Dr Claire Spottiswoode, a Royal Society University Research Fellow based at the University of Cambridge. "The honeyguide mother ensures her chick hatches first by internally incubating the egg for an extra day before laying it, so it has a head start in development compared to the host, and she also punctures host eggs when she lays her own. But some host eggs are overlooked or survive puncturing, and it is these eggs that precipitate chick killing by the young honeyguide has soon as they hatch."
Host parents are apparently blithely unaware of what is happening and, in the darkness of their burrows, even attempted to feed a honeyguide chick busy attacking their own young. Dr Spottiswoode added: "This behaviour is exactly analogous to that of young cuckoos, which hoist host eggs or chicks onto their backs and tip them over the rim of the nest. But because honeyguide hosts breed in tree holes or underground burrows, honeyguides can't eject host chicks and have instead evolved this highly effective killing behaviour to make sure that they alone monopolise the nest. Each time brood parasitism has evolved we see specialised adaptations for parasitic exploitation, which are no less astonishing for being sometimes rather gruesome."