Biological and environmental factors related to communal roosting behavior of breeding Bank Swallow (Riparia riparia)

Although communal roosting during the wintering and migratory periods is well documented, few studies have recorded this behavior during the breeding season. We used automated radio telemetry to examine communal roosting behavior in breeding Bank Swallow (Riparia riparia) and its relationship with biological and environmental factors. Specifically, we used (generalized) linear mixed models to determine whether the probability of roosting communally and the timing of departure from and arrival at the colony (a measure of time away from the nest) was related to adult sex, nestling age, brood size, nest success, weather, light conditions, communal roosting location, and date. We found that Bank Swallow individuals roosted communally on 70 ± 25% of the nights, suggesting that this behavior is common. The rate of roosting communally was higher in males than in females with active nests, increased with older nestlings in active nests, and decreased more rapidly with nestling age in smaller broods. Together, these results suggest that that communal roosting is limited by the thermoregulatory needs of the offspring. The rate of roosting communally and the total time spent away from the nest while roosting also decreased with humidity and low temperatures (total time only), supporting the conclusion that the thermoregulatory needs of both offspring and adults limit this behavior. Communal roosting was also restricted to dark nights, suggesting that the Bank Swallow may roost communally to avoid predation. Individuals also roosted communally and spent more time at the roosts as the breeding season progressed, suggesting that communal roosting may be a way of avoiding the growing number of ectoparasites at the colony or taking advantage of prospecting opportunities in the morning. The Bank Swallow is listed as Threatened in Canada, so understanding the factors that relate to communal roosting is essential for identifying the critical habitat of this declining species