Nest defense by Iberian azure-winged magpies (Cyanopica cyanus): do they recognize the threat of brood parasitism?

The ultimate goal of nesting is the production of young and therefore predation and brood parasitism may constrain reproductive success. Breeding azurewinged magpies (Cyanopica cyanus) were exposed to mounts of an sparrowhawk (Accipiter nisus) (i.e. an avian predator on adult and fledged chicks), a female great spotted cuckoo (Clamator glandarius) (i.e. a brood parasite that also preys on eggs and chicks), and a mistle thrush (Turdus viscivorus) (i.e. an innocuous control) at different stages throughout the breeding cycle. In addition, the nest defensive behaviour of the azure-winged magpie and the common magpie (Pica pica) (the current favorite host for the great spotted cuckoo in Iberia) were compared. Azure-winged magpies behaved more aggressively (i.e. higher intensity and shorter latency to an aggressive response) towards the brood parasite and the predator mounts than towards the control. The number of individuals defending a nest was higher when exposed to a predator than to a control or a brood parasite. Aggressiveness against the brood parasite mount was not higher at laying as compared to other breeding periods. Azure-winged magpies defended more intensely than magpies when facing a great-spotted cuckoo, although the number of birds attacking the decoy was similar for both species. Thus, azure-winged magpies appear to perceive a different level of threat and defend their nests accordingly. We discuss the relevance of these results in explaining the current absence of brood parasitism in Iberian azure-winged magpies.