Nepotistic mobbing behaviour in the Siberian jay, Perisoreus infaustus

Antipredator theory has rarely been invoked to explain why birds associate in kin groups. Instead, it has been argued that offspring delay dispersal in response to poor dispersal opportunities and changes in the social environment as a consequence of dispersal decisions have been overlooked. To test whether parents provide their offspring with nepotistic antipredator protection, we conducted a field experiment to investigate predator mobbing in the Siberian jay. We used models of a mounted hawk and owl, which represent the two main predators as confirmed by recoveries of killed radiotagged individuals. Kin groups mobbed the predator models for longer and uttered both more calls in total and more predator-specific calls than nonkin groups. However, not all categories of individual called equally. In particular, alpha males gave most calls, especially in the presence of their retained offspring. Unrelated immigrants behaved least cautiously and approached the models closer and for longer than any other individual category. Furthermore, jays mobbed the hawk model, their most dangerous predator, for longer and took fewer risks than they did when mobbing the owl model. Our experiments show that effort and risk taking while mobbing in the Siberian jay not only depend on the danger posed by the predator, but are also influenced by the social environment. Parents invest more in the presence of their offspring. We suggested that such nepotistic antipredator behaviour gives offspring an incentive to delay dispersal and that nepotism could therefore have a pivotal role for the evolution of kin group living in animals.