Asymmetric Antipredator Behaviour in a Mixed-Species Colony of Two Non-Mobbing Bird Species

Abstract

Avian species have a variety of antipredator strategies in response to predator threats of different levels. Mobbing behaviour is most common in colonial birds, although the defensive behaviour and interspecific relationships in mixed-species colonies composed of non-mobbing species are still unclear. In a mixed-species colony of Great Cormorants Phalacrocorax carbo and Grey Herons Ardea cinerea, we investigated defensive responses to potential avian predators and to actual avian predators. Our observations revealed that the birds distinguished between potential predators and reacted to particular predator species that could prey on large birds. Moreover, we found that the two colonial species showed different defensive antipredator behaviours: Herons exhibited aggressively defensive behaviours; whereas Cormorants, though vigilant, remained on the nest. To our knowledge this is the first report to suggest the possibility of commensalism in the Phalacrocoracidae, whereby Great Cormorants benefit from the defensive behaviour of Grey Herons.