Sex-specific shifts in natal dispersal dynamics in a reintroduced hihi population

Abstract

Sex-biased natal dispersal is prevalent in many avian species, with females typically dispersing further than males. We examined natal dispersal patterns in an growing reintroduced population of hihi (stitchbird, Notiomystis cincta) on Tiritiri Matangi Island in New Zealand. Hihi nest almost exclusively in the nest boxes provided throughout the island, and breeding locations have been recorded since the 1997–1998 season. The population grew from 16 to 170 birds in the 10 years since establishment in 1995, and also experienced substantial changes in adult sex ratio over this period. We calculated distances from natal to breeding locations to examine the dynamics of natal dispersal with sex, population density and sex ratio, and maternal and environmental effects. Overall, females dispersed significantly further than males, with permutation tests indicating that mean female dispersal distance was significantly higher than expected if they selected boxes at random, and male mean dispersal distance was significantly less than expected if they selected boxes at random. General linear mixed modelling also revealed sex-specific changes in dispersal behaviour over time that were correlated with changes in population density and sex ratio, and a strong effect of the maternal females' identity. The mean dispersal distance of males decreased with density, with the number of the juveniles in the cohort as the best correlate. We suggest that males are able to disperse further to gain the highest quality open territories at lower population densities, whereas dispersal distances become increasingly constrained by territory availability, and possibly variation in mating strategy, at higher densities. In contrast, female dispersal showed a weak positive density-dependent relationship, but was more strongly influenced by sex ratio, with dispersal distances decreasing with an increasing male bias in the population. We also found a strong effect of the maternal female that could not be explained solely by nest box location. Overall, these results imply a critical role of social context, including population size, density, and sex ratio in influencing natal dispersal dynamics in a closed, insular population of hihi, and have important conservation management applications for future translocation design and implementation for this endangered species.