EVOLUTIONARY PRECURSORS OF LONG-DISTANCE MIGRATION - RESOURCE AVAILABILITY AND MOVEMENT PATTERNS IN NEOTROPICAL LANDBIRDS

Seasonal movements are common in Neotropical forest birds. Species that engage in the largest movements, altitudinal and intratropical migrants, are predominantly frugivores or nectarivores of canopy/edge or dry habitats. Migration by such species may be necessitated by the high spatial and temporal variation in their resource base. We hypothesize that seasonal movements within the tropics predisposed these birds to migration out of the tropics. Indeed, most Nearctic passerine migrants are drawn from Neotropical taxa that exhibit altitudinal or intratropical migration. Conversely, taxa comprising the most sedentary group of Neotropical residents (understory insectivores) are poorly represented among Nearctic migrants. Further evidence of an evolutionary link between seasonal movements within the tropics and long-distance migration is provided by similarities in habitat use and diet between overwintering Nearctic migrants and closely related tropical residents. Both groups tend to be frugivorous/nectarivorous, inhabit "open" habitat, and engage in seasonal movements. The only difference in their movements is one of scale. These results help explain the behavior and ecology of Nearctic birds overwintering in the Neotropics.