Spatial and temporal dynamics of a Purple Martin pre-migratory roost

We used simultaneous WSR-88D radar (NEXRAD) and direct visual observations to investigate the spatial and temporal dynamics of a Purple Martin (Progne subis) pre-migratory roost in South Carolina. The timing of mass flights o f martins from and to the roost was related to levels of ambient light. Eac h morning, the birds first departed approximately 40 min before sunrise ind ependent of elate, with peak departures occurring about 10 min before sunri se. The time of evening flights was more variable, but peak movement of bir ds into the roost consistently occurred at sunset. Purple Martins exited th e roost in organized, annular departures (360 degrees) that were visible on radar up to 100 km away from the roost, but returned to the roost over an extended period in scattered flocks. During morning departures we recorded flight speeds up to 13.4 m/s. Radar echoes corresponding to martin flights were recorded farther from the roost, and flights from and to the roast occ urred later and earlier, respectively, in response to increased cloud cover . The departures of birds from the roost appeared to be displaced by winds aloft. At the peak of the roosting season in late July, the total roost pop ulation was estimated to be at least 700,000 birds.