Artificial Light at Night Promotes Activity Throughout the Night in Nesting Common Swifts (Apus apus)

Abstract

The use of artificial light at night (ALAN) is a rapidly expanding anthropogenic effect that transforms nightscapes throughout the world, causing light pollution that affects ecosystems in a myriad of ways. One of these is changing or shifting activity rhythms, largely synchronized by light cues. We used acoustic loggers to record and quantify activity patterns during the night of a diurnal bird - the common swift - in a nesting colony exposed to extremely intensive artificial illumination throughout the night at Jerusalem's Western Wall. We compared that to activity patterns at three other colonies exposed to none, medium, or medium-high ALAN. We found that in the lower-intensity ALAN colonies swifts ceased activity around sunset, later the more intense the lighting. At the Western Wall, however, swifts remained active throughout the night. This may have important implications for the birds' physiology, breeding cycle, and fitness, and may have cascading effects on their ecosystems.

Figure 1

Temporal activity patterns. Swift activity as represented by acoustic recordings. Multi-night mean (3-period moving average ± se, represented by dashed lines of the same color) of the number of files containing swift calls per 10-minute bins. Data were normalized by dividing each time-bin value by the maximum time-bin value (of the same colony), to allow a comparative representation of the different sized colonies (e.g. “Desert” colony, which was much smaller and produced an order of magnitude fewer calls). The bar at the top depicts day/night (sunrise/sunset) cycle (yellow = day, black = night). At all three low-ALAN sites swift activity ended around sunset, while at the high-ALAN site (Wall) activity continued throughout the night. Note that the evening peak starts at the same time at all sites, but activity drops under lower-ALAN conditions first.

Figure 2

Timing of activity relative to sunset and sunrise. Average (mean ± se) time in minutes between sunset and the end of recorded activity (dark gray), and between sunrise and the first recorded activity (light gray) (A), at the three sites, ordered by ALAN level (Desert < TLV < BI). Zero represents sunset for end of activity and sunrise for beginning. The lower ALAN intensity, the earlier the swifts cease their daily activity. This is not the case in the beginning of activity: while the two artificial sites (TLV & BI) follow the pattern (less ALAN = later beginning), the natural site with the least ALAN shows an intermediate value and the swifts there began activity earlier than would be expected. (B) Average (mean ± se) time in minutes between sunset and the beginning (light gray) and peak (dark gray) of the evening activity peak. The beginning of the peak was defined as the time-bin with at least 50% more recordings than the previous time-bin. Peak was defined as the time-bin with the highest number of recordings before midnight. While the evening peak begins at about the same time at all three sites, its peak is reached later the more intense ALAN is.