Light data from geolocation reveal patterns of nest visit frequency and suitable conditions for efficient nest site monitoring in Common Swifts Apus apus

Abstract

ABSTRACTCapsule: We developed a protocol for efficient monitoring of potential Common Swift Apus apus nest sites which considers variation in nest visit frequency across the breeding season and in relation to time of day and weather.Aims: To investigate patterns of nest visit frequency in Common Swifts in order to improve the efficiency and reliability of the monitoring of nest sites threatened by building renovations.Methods: We derived information on nest attendance from light data recorded by geolocators from ten adult Common Swifts during three breeding seasons (n = 686 individual sampling days) and analysed how nest visit frequency varied across the breeding season and in relation to time of day and weather.Results: The mean nest visit frequency was 5.63 visits per bird per day (0.32 visits per hour of daylight). The daily number of visits was highest at the beginning of July during chick-rearing. Moreover, it was positively correlated with temperature and negatively correlated with rainfall and wind speed. Nest visit frequency showed a distinct peak around sunset, while also being relatively high in the morning and around noon.Conclusion: We recommend monitoring potential Common Swift nest sites in Central Europe between the end of June and mid-July during good weather between 0.50 and 7.75 h after sunrise or between 3.00 h before sunset and sunset, when observation bouts of 0.5–2.0 h provide an encounter probability greater than 90%. Our study shows that repurposing geolocator light data – usually used to study bird migration – for investigating nest attendance in cavity-breeding birds can provide important information for bird conservation.