Social environment predicts alarm calling in Eurasian magpies

Abstract Animals in dangerous situations often use alarm calls to warn others. Variation in this behaviour across different social settings can serve to answer long-standing questions about the evolution of costly cooperative actions, since warned conspecifics may yield kin-selected, reproductive or mutualistic benefits to the caller. We experimentally tested how responses to predators vary according to differences in social settings in wild Eurasian magpies (Pica pica). Magpies called more and approached predator models more closely when they were with their partner. This effect persisted in the non-breeding period when no offspring were present any more, suggesting that magpies engage in anti-predator behaviour to warn or protect their breeding partner as well as their offspring and thereby gain both future and current reproductive benefits. In contrast, the low alarm call rates of lone magpies or flocks suggest that self-serving pursuit-deterrent alarm calling, quality advertisement, kin selection, or immediate mutualistic benefits (help in defence) do not function as drivers of alarm calling in this species. Our approach enabled us to test the various hypotheses on the evolution of alarm calling in a year-round territorial bird species, and suggests that long-term benefits of partner maintenance can outweigh short-term costs of risky alarm-calling behaviour.