Ecological determinants of<i>Tyrannus</i>flycatcher nestling growth at north- and south-temperate latitudes

Abstract

An organism's life history strategy is made up of a suite of physiological, behavioral, and ecological traits, which vary at both 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 fitness because growth speed could influence subsequent reproductive performance and survival. We studied the relationship between ecological factors (i.e. precipitation level and predation rate) and nestling growth patterns of 2 New World flycatcher species (Tyrannidae) of the genus Tyrannus (Fork-tailed Flycatcher [T. savana] and Scissor-tailed Flycatcher [T. forficatus]) breeding at south-and north-temperate latitudes. We tested the hypothesis that nestling growth rates are driven by nest predation rates and predicted that nestling growth rates would be higher in species experiencing higher nest predation rates. We also tested the hypothesis that nestling growth rates are related to precipitation levels (a proxy for food abundance) and predicted that nestling growth rates would be higher at sites with higher precipitation levels. Growth rate was not associated with predation rate, but it varied with precipitation level, with faster nestling growth rates during wet years for the Scissor-tailed Flycatcher 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 than predation. Additionally, the variation in growth rate we observed between wet and dry years indicates a high level of plasticity in growth rate in this group of insectivorous birds.