From Parasitism to Mutualism: Unexpected Interactions Between a Cuckoo and Its Host

Predation Favors Parasitism Parasitism in birds often results in ejection or starvation of the host's nestlings. Consequently, many host bird species have evolved protective behavior such as mobbing and parasite egg rejection. Curiously, some host species show no parasite avoidance behaviors; for example, the crow Corvus corone corone tolerates cuckoo chicks among its own brood. In a long-term study, Canestrari et al. (p. 1350 ) found that crow nests containing a cuckoo chick had lower rates of predation because the parasite's chicks secrete a noxious repellent substance. Overall, in years of high predation pressure, the presence of cuckoos improves the crow's breeding success, but when there are fewer predators around, parasitism reduces crow fitness.