Size matters: Chimney opening area predicts roost size in chimney swifts

Chimney swifts ( Chaetura pelagica ) are commonly found in urban areas, where they have experienced significant population declines despite their ability to use artificial structures like chimneys for roosting and breeding. This study examines how group size at roosting sites is influenced by chimney opening size, seasonality, and artificial light at night (ALAN). We hypothesized that larger openings would support larger groups, particularly during migration, due to increased space and accessibility, while smaller openings might be more suitable for breeding, offering enhanced protection from predators and the elements. We identified 59 roosting sites using eBird data and direct observations, measured chimney opening areas, and assessed light pollution using satellite data. Generalized linear mixed models revealed a significant positive relationship between chimney opening size and group size: larger chimneys tended to host bigger groups of swifts, especially during post-breeding migration when swifts form large communal roosts. Furthermore, in areas of high light pollution, we detected a temporal decline in group size over the study period, indicating that light pollution itself or factors associated with light pollution may be negatively impacting the species. These findings suggest that both chimney characteristics and environmental factors, such as light pollution, play critical roles in determining roosting group sizes of Chimney swifts, and should be considered in developing effective conservation strategies for this species, particularly in urban landscapes.