Comparison of Roost Use by Three Species of Communal Roostmates

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Abstract
Our previous studies of communally roosting European Starlings (Sturnus vulgaris) revealed that each bird fed daily for months on its own "diurnal activity center" (DAC) and commuted to a variety of nearby and distant roosts. Since these observations contrast sharply with the predictions of most foraging explanations for communal roosting, we wanted to determine if this DAC-centered roosting pattern occurred in other communally roosting species. In this study we used radiotelemetry to monitor feeding and roosting sites used by starlings, Common Grackles (Quiscalus quiscula), and American Robins (Turdus migratorious), three species that share communal roosts in central New Jersey [USA]. Our goals were to determine (a) whether avian species that roost together use the roosts in similar ways, and (b) when and why individuals change roosts. Foraging patterns were similar in all three species, individuals fed daily on their DACs for many weeks. Roosting patterns were similar for grackles and starlings; individuals switched among nearby and distant roosts. In contrast, robins always roosted near their DACs, changing both roosts and DACs at the end of the breeding season. Predation rates at roosts were extremely low and did not explain the use of patterns of large and small roosts. We argue that (a) DACs and DAC-centered roosting are probably widespread among communally roosting species, and (b) DAC-based individuals select roosts primarily on the basis of their proximity to good sources of food.