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.
Group size has been shown to positively influence survival of group members in many cooperatively breeding vertebrates, including the Lake Tanganyika cichlid Neolamprologus pulcher, suggesting Allee effects. However, long-term data are scarce to test how these survival differences translate into changes in group extinction risk, group size and composition. We show in a field study of 117 groups from six different colonies (three from two populations each), that group size critically influences these parameters between years. Within one year, 34% of the groups went extinct. Group size correlated positively between years and large groups did not go extinct. The latter were more likely to contain small helpers the subsequent year, which is a cumulative measure of the previous months' reproductive success. Finally, there was a tendency that large groups were more likely to contain a breeding male and female still a year after the first check. The breeder male size, breeder female size, and largest helper size did not influence these parameters, and also did not correlate with the sizes of these categories of fish after one year. This suggests that group size, and not the body size or fighting ability of group members, was the critical variable determining the success of groups. In total, seven groups had fused with other groups between years. To our knowledge, this is the first study showing long-term benefits of large group size in a cooperatively breeding fish. We discuss the importance of differential survival and dispersal of group members for the demonstrated group size effects.