Impact of marsh wrens on reproductive strategy of red-winged blackbirds

Abstract

Marsh wrens destroy redwing eggs and young and are a major cause of redwing nesting mortality in a brackish water marsh in Delta, B.C. Redwing nesting success increases with distance of redwing nests from marsh wren nests. In spite of higher wren nest densities in 1977, wrens had a relatively smaller impact on redwing nesting success in 1977 than in 1976, when wren nest densities were lower.Interference between redwings and marsh wrens has apparently evolved to reduce competition between these species through the spatial segregation of their nesting sites. As a consequence of different efficiencies of their interference mechanisms in vegetation of various densities, marsh wrens appear to be more successful in dense cattail whereas redwings are the more successful species in sparser vegetation.Marsh wrens have an important effect on redwing reproductive strategy. Contagious nesting by redwing females significantly reduces impact of wrens. Individuals failing to adopt this strategy have a negligible chance of reproductive success. The hypothesis is proposed that marsh wrens may have influenced the evolution of a clumped pattern of nesting by redwings.