Understanding the detrimental effects of anthropogenic light on nocturnally mobile animals is a long-standing problem in conservation biology. Seabirds such as shearwaters and petrels can be especially affected, perhaps because of their propensity to fly close to the surface, making them vulnerable to encountering anthropogenic light sources. We investigated the influence of light pollution on adult Manx Shearwaters Puffinus puffinus at close range in foggy conditions. We recorded collisions with a building at a breeding colony for six consecutive pairs of intervals in which the house lights were left on as normal for 135 seconds, then turned off for 135 seconds. The relationship between lighting condition and collision frequency was highly significant, with a collision rate in the presence of lighting around 25 times that in its absence. Our results show that birds were clearly affected by the lights, by being either directly attracted, or disorientated during flight close to the structure. This could have been due to the light source itself, or an indirect effect of the all-round reflective glow in the fog perhaps interfering with visual or magnetic control inputs on both sides of the bird simultaneously. Our results suggest a mechanism by which the screening of artificial lights close to shearwater breeding areas, at least during foggy nights, could lead to improved welfare and survival at breeding colonies.

Understanding the detrimental effects of anthropogenic light on nocturnally mobile animals is a long-standing problem in conservation biology (Montevecchi 2006; Gaston et al. 2013; Gaston et al. 2014). Artificial light at night can draw individuals from long distances, whilst repelling others, affecting many activities including foraging (e.g. Garber 1978; Frank 2009; Pereszlényi et al. 2017), reproductive behaviour (e.g. Miller 2006; Kempenaers 2010; de Jong et al. 2015; Russ et al. 2017), and daily, monthly or annual movements (e.g. Salmon 2003; Mathews et al. 2015; Rodríguez et al. 2017).

Amongst the most endangered groups of birds (Croxall et al. 2012), shearwaters and petrels (Procellariiformes) are especially badly affected by anthropogenic light sources and the grounding of their fledglings, in particular, has been studied in many parts of the world (Rodríguez et al. 2017). Manx Shearwaters Puffinus puffinus, which breed at island colonies predominantly around the British Isles, are classified as of Least Concern by the International Union for the Conservation of Nature (IUCN), but still they are often reported grounding in artificially lit urban areas or at other light sources particularly during the fledging period (Brooke 1990; Le Corre et al. 2002; Rodríguez et al. 2008; Miles et al. 2010; Archer et al. 2015). There are fewer studies on the grounding of adults on land, in this or other species, since adults usually constitute a small percentage of the individuals affected in any fallout (Telfer et al. 1987; Le Corre et al. 2002; Rodríguez & Rodríguez 2009).

Here we report a short experiment designed to test the instantaneous effect of anthropogenic house light on collisions with a man-made object, and outside of the fledging period when only adults are present at breeding colonies. Electric lighting is used to provide night service to the seasonal residents (a nightly summer maximum of 42 staff and tourists, of which up to 10 might stay in the Island Office) of the several buildings on Skomer Island National Nature Reserve (NNR), an internationally important Manx Shearwater breeding colony. Manx Shearwaters returning to the colony sometimes collide with the buildings and higher numbers of crashing seabirds are expected on cloudy and rainy nights (Jones 1980; Telfer et al. 1987). It may be that more birds visit breeding colonies in such conditions anyway, when there is less ambient light from the moon (Riou & Hamer 2008), but it is also possible that visual guidance in local manoeuvring flight is less effective and this contributes to collision risk. To determine whether there is a local effect of artificial light on collisions with the structure from which it is emanating, we conducted a very short experiment.

We would like to thank the Skomer and Skokholm Islands Conservation Advisory Committee, the Wildlife Trust for South and West Wales, and Natural Resources Wales.

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