Does Light Condition Affect the Habitat Use of Soprano Pipistrelles Pipistrellus pygmaeus at the Species Northern Extreme?

Abstract

This study explores the hunting habitat and activity patterns of the soprano pipistrelle Pipistrellus pygmaeus in relation to insect densities and light conditions during summer at 62°N in western Norway. Here, the first soprano pipistrelles emerge at several thousand lux and are common in woodland at more than 1000 lux. In this study, bats tagged with transmitters emerged from their roosts on average one hour before sunset and were airborne for more than five hours each night. During the first hours they always hunted in woodland, but shifted to hunt above the fiord during the night. This shift occurred on average 2 h and 25 min after evening emergence and 1 h and 30 min after sunset. In addition to using radio telemetry, bat contacts over the fiord were counted using ultrasound detectors and car transects. Simultaneously, insects were collected using suction traps and light levels were measured. There was a highly significant effect of light intensity on the number of bats hunting along the fiord. Predictions based on a second order polynomial generalised linear model (GLM) shows that soprano pipistrelles will start to hunt above the fiord when light levels drop below approximately 25 lux. It also suggests a slight reduction of insects as bat numbers increase along the shoreline. The GLM model explains approximately 92% of the variation in the dataset. Ultrasound recordings show that soprano pipistrelles attack far more prey per effort near the shores compared to areas further away. The results found in this study strongly suggest that habitat selection is a trade-off between food energy intake and other factors, e.g. predation risk.