Comparison of autumn and spring migration strategies of Neotropical migratory landbirds in northeast Belize
Migration represents one of the most vulnerable stages of a migrant's life cycle, but the strategies and stopover sites used by Neotropical migrants in Central America are not well known. We carried out constant-effort mist netting and conducted censuses along transects during one autumn (2007) and one spring (2008) migration in northeast Belize. We recorded more landbird migrant species in autumn (63) than in spring (54), and spring abundance was >25% lower for 88% of transient species. These differences in presence and abundance indicate that routes and stopover strategies vary between seasons and species. In autumn, fuel loads, calculated as any increase in mass above lean body mass (LBM), were generally small (mean = 5.9% LBM and 10.1% LBM for wintering and transient species, respectively) and fuel deposition rates and minimum stopover durations suggest that some individuals replenished energy reserves in our study area. Variation in autumn fuel loads meant that some individuals had reserves sufficient for flights >1000 km. Fuel loads were larger in spring for 16 of 17 species, and the mean spring fuel load for transient species (32.5% LBM) was sufficient for a flight from northeast Belize to North America without refueling. The similarity in spring passage times between northeast Belize and the Gulf Coast of the United States also suggests that energy reserves were not replenished in northeast Belize prior to crossing the Gulf of Mexico. We hypothesize that sufficient energy reserves are accumulated during spring stopovers in northern South America or elsewhere in Mesoamerica to allow migrants to fly directly to North America without refueling.