Abstract
Many species of swallows and martins congregate in large nonbreeding aggregations throughout the Americas. These roosts typically occur for several days to weeks in the same place during the same time of the year and disappear suddenly as the birds continue their migratory journeys. In the Amazon Rainforest, however, there are reports of large communal roosts of varying species composition throughout the year. Due to the high biomass density of these aggregations, we can systematically observe these tropical roosts using data collected by the operational S-band Doppler weather radar located in Manaus (3°08'56.0″ S, 59°59'29.1″ W) Using data collected by this radar over 2 years (2014, 2015), we describe the temporal and spatial patterns of roost size in the Amazon Rainforest, and compare it to a similar dataset collected in the Great Lakes region of North America, where swallows and martins form pre-migratory roosts. Our findings confirm that roosting activity occurs throughout the year in the region around Manaus, and thus likely gather multiple species of swallows and martins. The peak of roosting activity in both years occurred from January to May, when observations on the ground suggest that roosts are predominantly Purple Martin aggregations. We found that the average daily number of birds in roosts in Manaus in 2015 is up to 7 times larger than what was observed in the Great Lakes, even though the area of the latter is 8.7 times larger than the area sampled around Manaus. Our findings highlight the significance of the Amazon Rainforest for swallow and martin populations. Because this region hosts migrating individuals from the Nearctic-Neotropical and the Austral migratory systems, resident species may share these roosts with populations from both systems during separate times of the year, creating an indirect link between the two poles of the Americas.