Smelly cuckoos protect crow chicks

THEY are usually birds of ill omen. Cuckoos are notorious for taking over other birds’ nests and killing their chicks. But one species benefits its hosts: its stink repels predators. Carrion crows (Corvus corone corone) often play host to chicks of the great spotted cuckoo (Clamator glandarius). Daniela Canestrari of the University of Oviedo in Spain and colleagues found that crows with a cuckoo chick in the nest were 40 per cent more likely to raise one of their chicks to adulthood, compared with crows not hosting cuckoos. Hosts only had this edge if predators were active, and did so even though their nests had fewer crow fledglings on average (Science, doi.org/rz8). The cuckoo chicks emit a foul  fluid from their cloacas. “It is really disgusting,” says Canestrari. “It’s pungent, produces a burning sensation in the throat, and looks  like rotting matter.” She tested its effect on predators by offering meat smeared with it to eight cats. Only one bit into the meat, whereas most would eat untreated meat. “Cuckoos are not always the villains we think they are,” says Ros Gloag of the University of Sydney.