Population Regulation Of A Songbird In The Non-breeding Season: A Test Of Buffer And Crowding Effects
Animal populations are limited by their environment and interactions with one another, the latter proportional to density. How density-dependent mechanisms regulate populations is poorly understood, particularly for migratory animals. Winter, or non-breeding, mechanisms remain particularly poorly understood for almost all migratory bird species. This dissertation tested the hypothesis that American Redstarts (Setophaga ruticilla) are regulated both within and between habitats in the winter by two mechanisms, a crowding effect and a buffer effect. As population size increases, crowding increases competition for space and resources and more individuals are relegated to lower quality habitats and forced into a transient behavioral strategy, which buffers high quality habitats from negative feedback of density. My study was carried out in Jamaica, where redstarts occupy diverse habitats. I found that population size varied among habitats, but accounting for both territorial and transient individuals was critical for accurate assessment of this variation. Ecological conditions drive differences in habitat suitability and redstart density at multiple spatial scales. Regionally, redstart survival, density, and numbers of transient individuals distinguish inland from coastal populations, consistent with differences in ecological conditions (e.g. rainfall). Locally, timing of spring departure, as well as age and sex structure distinguish neighboring habitats, consistent with increased competition for food. When population size increased over the years of this study, changes in local density were similar among habitats, and thus not the result of a buffer effect. However, increasing densities decreased food availability, which corresponded with negative feedback on departure timing and territorial behavior, i.e. more transients, in poorer habitats. This supported a crowding effect on spatial behavior (territoriality) and late-winter condition of individuals relegated to low suitability habitats. High quality inland habitats appear to buffer individuals from the effects of changing population abundance, as well as ecological conditions, evidence of a regional buffer effect.