Interacting moonlight, weather, and sex influence foraging in a nocturnal insectivore Caprimulgus europaeus

Abstract

Animals use a variety of environmental cues to identify profitable conditions and increase their foraging efficiency. Identifying environmental correlates of foraging behaviour can help to understand the drivers of animal foraging decisions, and the potential consequences should conditions change. We used GPS tracking devices to collect data from 2015 to 2018 on movement behaviour of 34 individual European nightjars Caprimulgus europaeus, which forage exclusively at night on aerial invertebrates. To understand how they allocated their time, and whether their behaviour was influenced by the lunar cycle, weather conditions, and environment, we analysed the environmental factors that influenced foraging within and across multiple nights. Nightjar fine-scale changes in behaviour were primarily driven by different aspects of the lunar cycle, and cloud cover, with a lesser influence from habitat type and temperature. At an hourly level, nightjars responded to cyclical cues most strongly, foraging more when lunar light levels were high and the moon was close to the earth. Birds foraged for longer when the moon was between 60 and 90% illuminated, but the strength of this relationship varied with lunar distance. Foraging was also influenced by the amount of cloud present and lunar altitude. Nightjars concentrated their foraging in nautical twilight, when light levels were still sufficient, and temperatures likely still higher, thus providing efficient foraging via increased visibility, as well as better prey availability, supporting the reduced effort hypothesis. Male nightjars foraged significantly more than females, representing their differing breeding roles and potentially their energetic requirements. We suggest that our results indicate a trade-off between prey availability and visibility that may reflect allocating more time to foraging in conditions that promote reduced effort and increased efficiency.