Rescue behaviour in a social bird: Removal of sticky 'bird-catcher tree' seeds by group members

Rescue behaviour is a special form of cooperation in which a rescuer exhibits behaviours directed towards averting a threat to an endangered individual, thereby potentially putting itself at risk. Although rescue behaviour has been well-documented in experimental studies on rats and ants, published cases in other non-human animals are rare. Here, we report observations of rescue behaviour in the cooperatively breeding Seychelles warbler (Acrocephalus sechellensis). In this species, individuals sometimes become entangled in seed clusters of 'bird-catcher trees' (Pisonia grandis). Just one or a few of these sticky seeds can prevent Seychelles warblers to fly and may lead to mortality. In four cases, individuals were observed displaying behaviour aimed at removing sticky seeds from the feathers of an entangled individual belonging to their group. Intriguingly, the rescuing individuals engaged in this behaviour despite potentially risking entanglement. To our knowledge, this is the first recorded case of rescue behaviour in birds.