Patterns of nocturnal bird migration in southern Mexico

Abstract

The Isthmus of Tehuantepec is the narrowest landmass between the Gulf of Mexico and the Pacific Ocean. It is known as an important stopover site and migration corridor for Nearctic-Neotropical migratory birds. We characterized nocturnal bird migration along the Pacific side of the isthmus with marine radar, and identified species with diurnal point counts during 5 fall seasons (2009–2013) at operational and proposed wind farms. We found significant differences in migratory traffic rates, flight directions, and flight altitudes among years and sites, but we also found patterns of similarity including flight directions heading to the southeast each season; migratory traffic rates peaking 3–4h after sunset and decreasing thereafter; flight altitudes increasing throughout the night; seasonal mean flight altitudes fluctuating between ∼520 and ∼720m above ground level (m agl); and <7% of detected targets flying below 100m agl. Our data suggest that migratory activity initiates around mid-September and peaks in mid-October. We identified 58 bird species of nocturnal migrants. Studies of nocturnal bird migration are important for understanding potential impacts from wind farms.