Non-breeding competition between migrant American Redstarts (Setophaga ruticilla) and resident Adelaide's Warblers ( Dendroica adelaidae) in the Guanica Biosphere Reserve, southwest Puerto Rico

Evolutionary processes increase fitness by reducing overt competition among species through morphological character displacement, habitat preference, behavioral adaptations, and divergence in use of resources. However, migrants face a different set of ecological pressures in their breeding, non-breeding and passage/stopover habitats. As a result, they may evolve a compromised set of traits that are not optimal in any of these ecosystems and competition may be more easily detected between migrants and residents than among residents.;I tested the hypothesis that migratory American Redstarts (Redstart; Setophaga ruticilla) and resident Adelaide's Warbler (Adelaide's; Dendroica adelaidae) compete for food in a dry forest of southwest Puerto Rico. My study was conducted during the non-breeding season, when the only plausible resource that might be in limited supply was food (arthropods). The two species have broadly similar foraging methods, and arthropods are limiting in similar ecosystems on other islands.;In addition, I determined reliable ageing and sexing criteria for Adelaide's, which allowed me to control for age and sex differences. I also described their molt patterns, which indicated the time of year when these criteria could be dependably applied.;Competition will not occur unless food is limiting, so I monitored arthropod biomass through two non-breeding seasons. In the drier of the two years, leaf fall was pronounced in all vegetative layers and total arthropod biomass declined. In contrast, during the wetter year, leaf fall was largely limited to the shrub layer and total arthropod biomass remained fairly constant. Little mortality was observed in either bird species throughout the non-breeding season. However, the body condition of both species declined during the drier year, while they managed to maintain body condition in the wetter year. Decreased body condition may delay departure of the migratory Redstart, which is correlated with increased mortality during migration and reduced reproductive success. Similarly, decreased body condition in the resident Adelaide's may delay their response to conditions favorable for breeding, limiting their reproductive success.;Even though arthropods are limiting, Redstarts and Adelaide's will not compete unless they forage in the same places using the same methods of attack. When both species were present, they had a very high degree of overlap in both foraging location (type of vegetation, relative location within tree canopies and height from ground) and type of attack (substrate and attack method), both when assessed at the level of the population and at the level of individuals. Thus, they are indeed using the same limiting resources, and should be competing.;The high degree of similarity in foraging niches when both species are present indicates that they do not partition food resources. However, Redstarts might still be preventing Adelaide's from using preferred resources. Therefore, I determined whether Adelaide's shifted their foraging niches when Redstarts left, using Adelaide's without Redstart neighbors as a control for any seasonal changes. Adelaide's did not shift their foraging niche, indicating that depletion competition does not occur.;The two species could also defend territories against the other species. However, detailed territory mapping showed that interspecific overlap of territories was significantly greater than the intraspecific territory overlap of either species. In addition, Adelaide's did not shift territories when Redstarts left, indicating that Redstarts were not restricting the space use of Adelaide's. Thus, interference competition is not present.;I also conducted a behavioral experiment, where I simulated territorial intrusions to determine whether each species was aggressive towards the other species. Aggressive behaviors have high costs, and should be used only if the benefits of aggression outweigh these costs. Both species were significantly more aggressive to the other species than to a non-competitive control, with Adelaide's being the dominant species.;Adelaide's and Redstarts do seem to be competing for food, but not through niche partitioning or interspecific territoriality. Instead, the evidence suggests that Redstarts listen to the frequent vocalizations of Adelaide's, and forage in areas of their territory that reduces the risks of confrontation at any given time: they use temporary competition refuges. Redstarts are not territorial where Adelaide's densities are high, suggesting that Redstarts may be vagrant in these areas, or avoid them entirely. I conclude that competition does occur between these species, and is mediated through aggressive confrontations and the behavioral flexibility of Redstarts.