Chemical defence in avian brood parasites: production and function of repulsive secretions in common cuckoo chicks

The use of active chemical defence against predators is relatively rare in birds. Among others, it has been reported for some members of family Cuculidae whose chicks, when threatened, expel dark foul‐smelling liquid from their cloaca. Apart from the brood parasitic great spotted cuckoo Clamator glandarius , however, this phenomenon has not yet been systematically studied in any other cuckoo species. Here we investigated the repellent behaviour in the evicting brood parasite, the common cuckoo Cuculus canorus , parasitizing the great reed warbler Acrocephalus arundinaceus . We explored whether production of secretions varies with chick age or size, and tested its presumed repellent function against various types of predators. We found that the production of secretions commenced at the age of approximately eight days, then gradually increased and decreased again shortly before fledging. Furthermore, we experimentally confirmed a more intensive repellent effect of the secretions on mammal predators than on avian predators, such as raptors and owls. The secretions have, however, no effect on corvid predators, probably because these scavengers often consume malodorous food. Further experimental studies together with phylogenetic comparative analyses are needed to elucidate the origin and function of this intriguing phenomenon both in parasitic and non‐parasitic cuckoos.