Light pollution hampers recolonization of revitalised European Nightjar habitats in the Valais (Swiss Alps)

Increasing light emissions caused by human activities have been recognized as a major threat for nocturnal animals. In Switzerland, the European Nightjar is a rare bird, decreasing in numbers since the 1970s, and is therefore highly threatened. The last breeding population occurs in the canton Valais. Initial expert-based conservation measures on formerly inhabited breeding sites were successful until 2000, however recent additional measures have failed. Nightjars are highly sensitive to light due to their special retina adapted to living in semi-darkness. We hypothesized that food availability, mainly moths, is not a critical limiting factor, but that artificial light emissions prevent successful foraging as well as recolonizing revitalised breeding habitats of the nightjar. To test this hypothesis, we used light trapping data of moths from the last 30 years to evaluate food availability and compared light emission on abandoned versus still-occupied breeding sites. Abundance of larger moths did not change significantly over the last 30 years, and smaller moths even increased in abandoned as well as in still-occupied nightjar habitats. However, light emission was two to five times higher in abandoned compared to still-occupied sites. These results suggest that increasing light emission during recent decades has exceeded tolerable levels for this highly specialized night bird. Authorities of the canton Valais should therefore order a reduction in light emission near nightjar habitats by replacing bulbs currently in use with customized LED or broad-spectrum lamps low in white and blue light, and assign remaining nightjar habitats as areas of complete nocturnal darkness, thereby also protecting other threatened nocturnal animals, including moths.