A megaroost of Purple Martins in the Amazon
Eliene Arruda, The National Institute of Amazonian Research
Wintering is the least known part of the annual cycle of many migratory birds from North America, including the Purple Martin (Progne subis). During their wintering season in South America, martins gather in large numbers every night at roosts. Such areas provide opportunities to study this declining species on the wintering grounds. We studied a winter roost of P. subis near the city of Manaus (Amazonas, Brazil) to determine the number of individuals, age and gender composition, roost permanence, and general behavior. We estimated numbers of martins by making morning counts at roost departure, with observers at different positions estimating flux of birds per second. At night we captured and tagged birds with Motus radio transmitters to sample their population composition and relate their roost permanence to relevant body variables such as weight, fat, molt, muscle, sex and age. We estimated the presence of about 250,000 birds every night between the months of February and March of 2020. Analysis of 59 tagged birds (21 in 2019 and 38 in 2020) indicate that birds remain at the roost for only a few days, apparently related to molt stage and fat stores. In general, birds with greater fat stores and completed flight feather molt left the roost sooner, suggesting that the roost serves as a staging ground prior to spring migration, and that many more martins use the roost over the active period than are present on any given night. The only confirmed predators detected at the roost were Peregrine Falcons (Falco peregrinus) at dawn and dusk and Stygian Owl (Asio stygius) at night. Arrival and departure behaviors of martins at the roost appear to be adapted to avoid falcon attacks. Future studies will attempt to determine the total number of birds that use this roost for staging throughout the season and so its importance for the global Purple Martin population.