Spatial and temporal variation in maternity roost site use of common pipistrelles Pipistrellus pipistrellus (Mammalia: Chiroptera) in Rotterdam

The global trend of urbanization has a negative impact on biodiversity, but affects organisms in a species-specific way. Common pipistrelles Pipistrellus pipistrellus have adapted successfully to (sub)urban areas and nowadays generally find their roost within man-made structures. Nevertheless, they experience threats from urbanization, especially due to the urge to improve thermal insulation of houses, a process during which many existing and potential roosts disappear in The Netherlands. Increasing the knowledge on pipistrelle behaviour at roost sites is important to improve protective measures that are taken prior to renovation and demolition works, and therefore essential for pipistrelle conservation. Moreover, newly acquired knowledge may benefit construction of bat-friendly buildings. In this study, we monitored two pipistrelle maternity colonies for 11 weeks in a pipistrelle-rich city (Rotterdam, the Netherlands). Main objectives were to describe phenology and spatial and temporal variation in maternity roost site usage. The use of a high-resolution thermal video camera herein provided unique insights in specific behaviour of this species. Our results show that just a single building or building complex already provides plentiful space for an entire maternity colony throughout the maternity period (April-July). Individuals in both maternity colonies frequently switched to new roost openings within the same building (complex). An often assumed preference for sun-lit or indoor-heated parts of buildings was not found. Both maternity colonies were already established from April onwards, considerably earlier than generally assumed. Pipistrelle emergence behaviour related to daylight, with an optimal moment of emergence at 36 minutes after sunset. Average onset of emergence was 17 minutes after sunset, and appeared to be earlier under lower wind speed or higher temperature conditions outside the roost. Average end of emergence was 55 minutes after sunset, with mean duration of the process being 35 minutes. Duration length was not related to the total number of bats emerging, showing that pipistrelles emerge faster when more individuals are to leave from the same opening. Bats often emerged in small clustered groups within a time frame of only a few seconds. Clusters were also sometimes formed by joining passing individuals from adjacent openings. Our study provides detailed insights into the behaviour of pipistrelles during the maternity period, particularly with respect to roost utilization and emergence timing. Moreover, our study challenges certain assumptions that currently serve as guidelines for taking protective measures. Ultimately, our results contribute to the knowledge on common pipistrelle conservation and suggest that alterations to current standard protocols are needed to safeguard their persistence in anthropogenic landscapes.