Female-biased dispersal in a spatially restricted endemic island bird

Abstract

Dispersal of individuals before their first breeding attempt or between subsequent attempts facilitates spatial and temporal gene flow within and among populations. However, in species on oceanic islands, dispersal is often restricted to a single island, and thus, the risk of inbreeding is particularly high in those small, closed, and isolated populations. One of the mechanisms that may prevent inbreeding within island populations is sex-biased dispersal, which results in close kin of the opposite sex not being in the same area for breeding. In this study, we investigated dispersal patterns, and their costs and benefits, in the Chatham Island black robin, a small passerine confined to two small islands. We found that black robins practice a resource defense mating system as male black robins were more likely to divorce than change territory between breeding seasons. Natal dispersal was female-biased in both the proportion of birds dispersing and the distance dispersed. Bird density in the natal year increased the proportion of birds dispersing in both sexes. Breeding success was reduced for females after natal dispersal. Breeding dispersal was rare and female-biased in proportion only. Regardless of sex, black robins were more likely to disperse after losing a mate, but females dispersed further than males. This study suggests that in closed populations of island species with limited habitat, sex-biased density-dependent dispersal may be a mechanism that minimizes inbreeding.

Significance statement

Dispersal of individuals facilitates gene flow within and among populations. However, in species on oceanic islands, dispersal is often restricted to movement within a single island. Few studies have investigated dispersal and its costs and benefits in spatially restricted species. Here, we studied the endangered Chatham Island black robin, a songbird confined to two small islands. We found that on an island, where habitat is extremely limited, males are highly territorial and they are more likely to change partners than territories. Bird density in the natal year forces both young males and females to move, but females are more likely to disperse and move further than males. Sex-biased density-dependent dispersal may be a mechanism that minimizes inbreeding. This is particularly important for threatened island endemic species where maintaining high genetic diversity ensures the population’s long-term viability.