AbstractsBiology & Animal Science

In the New World, most research on bird migration has been carried out in North America, which has led to the unfortunate situation that in most cases, evolutionary hypotheses have been both formulated and tested in the same migratory system, which is arguably not a true test of hypotheses. A much more logical approach would include tests of hypotheses in an evolutionarily related set of species that migrate within a different migratory system. Neotropical austral migration, in which birds migrate wholly within South America, provides such a system. I tested hypotheses on the regulation of partial migration by studying a population of Tropical Kingbirds (Tyrannus melancholicus) at a study site in eastern Bolivia. I found that the non-breeding season at the site is characterized by much lower food abundance for kingbirds than the breeding season, suggesting that environmental constraints are in place to create partial migration in the population. I employed maximum-likelihood theory to model the probability of migration by Tropical Kingbirds at my Bolivian study site. I show that partial migration does exist in the population, and by including demographic, life-history and morphological covariates in the model, I show that the hypotheses tested (the Dominance and Body size hypotheses) are not supported in the Tropical Kingbird partial migration model, possibly because these hypotheses have been developed for bird species that inhabit a colder continent (North America), and on granivorous species (e.g., sparrows, finches). I present a new hypothesis that accounts for these differences and that could be typical of many if not most Neotropical austral migrant passerines. This research provides some of the first quantitative measures of the mechanisms driving bird migration in South America. The results indicate that models on the regulation of bird migration primarily developed in the Northern Hemisphere are likely to be inadequate for many species migrating within South America, especially at tropical latitudes.