The role of behavioral dominance in structuring patterns of habitat occupancy in a migrant bird during the nonbreeding season

Several species of territorial migratory birds exhibit sexual habitat segregation on their wintering grounds, with some habitats containing mostly males and others mostly females. The objective of this study was to determine if in the American redstart (Setophaga ruticilla) in Jamaica habitat segregation is due to social mechanisms or due to sex-specific habitat specialization. I used habitat-specific patterns of arrival by young males and females, observations of territorial displacements, removal experiments, and simulations of territorial intrusions to differentiate between these two mechanisms. Redstarts were studied in two habitat types, a male-biased mangrove forest and a female-biased scrub habitat. In autumn, male and female hatch-year redstarts initially settled in equal numbers in each habitat, and segregation of the sexes occurred gradually and mostly later in the arrival period. This shift corresponded with an increase in density of older birds and an increase in territorial displacements. Removal experiments showed that vacancies in male-biased habitat were filled more rapidly and with greater frequency than those in female-biased habitat and that vacated male territories in mangrove were replaced more often by females than by males. Simulations of territorial intrusions and analyses of body size indicated that levels of aggression and body size of both males and females were greater in mangrove habitat, suggesting that these factors may be important in determining the outcomes of dominance interactions. I conclude that patterns of sexual habitat segregation in redstarts are structured by the dominance behavior of older and more dominant individuals, and these are mostly males.