Population decline of a long-distance migratory passerine at the edge of its range: nest predation, nest replacement, and immigration

The relative importance of predators and resources (i.e., food) for the dynamics of migratory bird populations is poorly known. Resource availability may be more likely in resource poor environments, but given that nest failure in most systems is due mainly to predation, predator effects may predominate. We document a rapid decline of an isolated Eastern Kingbird (Tyrannus tyrannus) population breeding in the Great Basin Desert of eastern Oregon, USA, and evaluate whether it was driven by limited food resources (water availability ~ food), nest predation, or first-year or adult return rate (RRJ and RRA, respectively) that reflect nonbreeding season events. Most nests failed (~68% of nests) due mainly to nest predation (>90% of failures); nestling starvation was rare. Bioyear precipitation (October-April), breeding season precipitation, and river flow all varied widely but none could account for annual variation in either nest success (NS) or fledging success of successful nest (FSSN)...