Dynamics of an urban Chimney Swift (Chaetura pelagica) roost system during autumn migration

Although urbanization can lead to habitat loss and biodiversity decline, it has also helped certain declining species recover by providing resources such as food or shelter. Chimney Swifts (Chaetura pelagica) are migratory aerial insectivores that adapted to use masonry chimneys as nesting and communal roosting sites after European colonization and subsequent widespread forest loss. These structures are now becoming obsolete and are being removed or capped, which again threatens the habitat availability for this declining species. In this study, we describe the autumn roosting dynamics of a system of urban roost sites in Asheville, North Carolina, USA, a fast-growing mid-sized city situated in the central Atlantic migratory flyway. Using historical community science records, we first compiled a list of chimneys within city limits that were known to be used as communal roosting sites during the autumn roosting season. We measured physical and environmental characteristics of these chimneys and related them to current and historical patterns of roost use. We found that a combination of chimney height and limited tree canopy cover within 50 m of the chimney explained much of the variation in maximum roost size. In the autumn of 2020, we surveyed most of these sites, many of them several times throughout the season, to understand site-specific use, timing, and roost size during the migration season. Roost sites were used sequentially rather than concurrently throughout the migration season: some roosts formed and dissipated early in the study period. other roosts did not form until later in the season and were larger than the earlier roosts. and sonic were occupied continuously until the last swills departed the study area. Our study shows that (1) post-breeding and migratory swifts may choose communal roost sites based on certain characteristics, and (2) these roost sites may serve different roles for different populations of swifts during the autumn migration season. which has conservation implications for this declining species.