Comparison of nest defence behaviour between two associate passerines

Nest predation is one of the most important factors limiting reproductive success, and antipredator behaviour can significantly reduce the loss of avian broods. I carried out field experiments on two sympatric passerines: the barred warbler and the red-backed shrike. Many authors have described the protective nature of nesting association between these species. However, we have little knowledge about the true nature of the relationships between associates. I examined (1) whether barred warblers and red-backed shrikes respond differently to an avian predator, and (2) whether males and females differ in the intensity of nest defence. Decoys of a known nest predator and a non-predatory control species were used to examine the types and relative intensity of parental response. I measured behavioural responsiveness by recording aggressive behaviour toward each model during the nestling period. Barred warblers and red-backed shrikes showed considerable variation in their response. Warblers more vigorously defended their own territories than shrikes. No differences between the sexes in antipredator behaviour in red-backed shrike were found. By contrast, in barred warbler, male was more involved in nest defence. The experimental tests provide evidence that these two species are able to differentiate between a predator and non-predator species.