CONICET | Buscador de Institutos y Recursos Humanos

An organism?s life history strategy is made up of a suite of physiological, behavioral, and ecological traits, which vary atboth the interspecific and intraspecific levels in accordance with selective pressures operating on individuals. For birds,2 primary ecological factors have been proposed to explain intraspecific and interspecific variation in nestling growth:nest predation and food availability. Individual nestling growth rates have important consequences for overall fitnessbecause growth speed could influence subsequent reproductive performance and survival. We studied therelationship between ecological factors (i.e. precipitation level and predation rate) and nestling growth patterns of 2New World flycatcher species (Tyrannidae) of the genus Tyrannus (Fork-tailed Flycatcher [T. savana] and Scissor-tailedFlycatcher [T. forficatus]) breeding at south- and north-temperate latitudes. We tested the hypothesis that nestlinggrowth rates are driven by nest predation rates and predicted that nestling growth rates would be higher in speciesexperiencing higher nest predation rates. We also tested the hypothesis that nestling growth rates are related toprecipitation levels (a proxy for food abundance) and predicted that nestling growth rates would be higher at siteswith higher precipitation levels. Growth rate was not associated with predation rate, but it varied with precipitationlevel, with faster nestling growth rates during wet years for species living at north-temperate latitudes. Among species,similar growth rates were found during wet years. These results indicate that, at least as proximate causes,precipitation explains intraspecific and interspecific growth rate variation in Tyrannus species to a larger degree thanpredation. Additionally, the variation in growth rate we observed between wet and dry years indicates a high level ofplasticity in growth rate in this group of insectivorous birds