Experimental study on the role of intra- and inter-specific competition in the evolution of nest-destroying behavior in marsh wrens

Abstract

Marsh wrens attack nests with eggs and young of other birds, including conspecifics. This study examined the relative role of interference from conspecifics and red-winged blackbirds in the evolution of the marsh wren nest-destroying behavior by giving male wrens a choice to attack a redwing or marsh wren nest. The combined data on wren responses from 4 years showed that male marsh wrens exhibited stronger responses to redwing than to wren nests, which suggests that in the study area the interference from blackbirds is usually more important than that from other marsh wrens. However, the between-year analysis, relating the intensity of marsh wren responses to nests of the two species to the intensity of intra- and inter-specific interference, indicated that marsh wrens adjust their responses to the prevailing type of interference. Thus marsh wrens attacked the redwing nest first when interspecific interference was more important, but their resonses to the wren nest became stronger with increasing intraspecific interference. These data support the idea that intense intraspecific competition as well as the interference from blackbirds have played an important role in the evolution of nest-destroying behavior by marsh wrens.