Territorial social behavior of wintering Nearctic—Neotropic migrant songbirds places males and females in direct conflict over access to winter space and resources. Outcomes of this intersexual competition can vary by species and habitat, but information has been collected for only a small subset of migrant species. We investigated the available food resources, sex ratios, and body condition of territorial Bicknell's Thrushes (Catharus bicknelli) wintering in the Dominican Republic between 1999 and 2008 at two ecologically distinct wet-forest sites, one in high-elevation cloud forest and the other in mid-elevation rainforest. Arthropod abundance was greater in cloud forest habitat, which was occupied by proportionally more males, the larger-bodied sex (74% male). By contrast, both sexes occurred at parity in rainforest habitat (53% male), where soft-bodied fruit was the predominant dietary resource. Body condition of cloud forest males was comparable to that of rainforest males, but cloud forest females were in poorer body condition than rainforest females. Females at the cloud forest site may face a greater likelihood of agonistic interactions with larger-bodied males and the thermoregulatory demands of roosting in colder night temperatures (0–12°C). We suggest that there are sex-specific advantages of wintering in these two habitats and that both are critical to supporting the full demographic structure of Bicknell's Thrush populations. Rainforest habitats, which are highly vulnerable to agricultural development in the Dominican Republic, may be particularly important to female survival during the winter period.
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Vol. 129 • No. 4