Seasonal passerine migratory movements over the arid Southwest

Biannually, millions of Neotropical and Nearctic migratory birds traverse the arid southwestern US-Mexico borderlands, yet our knowledge of avian migration patterns and behaviors in this region is extremely limited. To describe the spatial and temporal patterns of migration, we examined echoes from weather surveillance radar sites across the American Southwest from southern Texas to southwestern Arizona during spring 2005 and 2006 and fall 2005. After taking steps to identify radar echoes dominated by birds, we determined migrants’ speeds, directions, and altitudes. Our results show that in spring, migrants generally fl ew lower and faster than in fall, although much of this overall pattern may be driven by higher fall altitudes and higher ground speeds at some of the easternmost sites in the borderlands. Seasonal differences in migrants’ altitudes can be partially explained by seasonal differences in the altitudes of favorable winds. Seasonal differences in migrant ground speeds might arise for many reasons including variation in winds aloft or the presence of naïve hatchyear birds in fall. In addition, migrating bats may also be present throughout the region in varying degrees in radar data. Flight directions across the region were generally north in spring and south in fall, but also were consistent with the premise that songbird migration in North America is comprised of distinct regional migratory systems.