The Evolution of Avian Migration Systems between Temperate and Tropical Regions of the New World

Abstract

The Nearctic-Neotropical bird migration system is one of the major phenomena of vertebrate ecology and evolution. The origin of this system has been accompanied by a coevolutionary adjustment of migratory species with resident species in both temperate and tropical bird communities. Potential selective pressures for migratory behavior are varied, and include extreme weather conditions, thermal and photoperiodic regimes, changes in seasonal availability of specific resources, intraspecific and interspecific competition, and predation or parasitism. Several general theories of the origin of migration incorporate these selective pressures. These include the theories of increasing seasonality, optimal energy budget, intraspecific and interspecific competition, predation and food supply, and time allocation. None of these, as outlined to date, is able to account fully for the complex Nearctic-Neotropical migration system. An integrated time-allocation and competition theory is suggested, in which the Mexican Plateau and arid southwestern United States act as a staging area for the evolution of migration. In this region, where environmental unpredictability favors the reduction of site tenacity, various forces select for the movement of birds into neighboring areas with more favorable seasonal conditions, creating partial migrants. The increased seasonality of climate, the speciation of geographically isolated migrant population segments, and the elimination of permanent resident population segments by interspecific competition lead to disjunct breeding and nonbreeding ranges. It is assumed, therefore, that species migrating between the temperate zone and the tropics have populations that are resource-limited throughout the year. By virtue of the complementarity of ecological adjustments in breeding and nonbreeding seasons, I also infer that these migrants impose year-round population limitation on the members of the resident temperate and tropical bird communities.