The influence of artificial light at night and polarized light on bird-building collisions

Collisions with buildings annually kill up to 1 billion birds in the United States. Bird-building collisions primarily occur at glass surfaces: birds often fail to perceive glass as a barrier and appear to be attracted to artificial light emitted from windows. However, some aspects of avian vision are poorly understood, including how bird responses to different types of light influence building collisions. Some evidence suggests birds can detect polarized light, which may serve as a cue to assist with migration orientation and/or detect water bodies. Dark, reflective surfaces, including glass, reflect high degrees of polarized light, causing polarized light pollution (PLP). However, no studies have analyzed the relationship between bird collisions and PLP reflected from buildings. Additionally, while artificial light at night (ALAN) is frequently implicated as a major factor influencing bird-building collisions, few studies have analyzed this relationship. We investigated both types of light pollution-PLP and ALAN-and their association with bird collisions at 48 facades of 13 buildings in Minneapolis, Minnesota, USA. We found that the area of glass emitting ALAN was the most important factor influencing collisions, and that this effect of ALAN was independent of overall glass area; this result provides strong support for turning off lights at night to reduce bird-building collisions. Although we found no relationship between PLP and collisions, additional research is needed to better understand bird responses to polarized light. Fully understanding how different aspects of light influence bird-building collisions can inform conservation efforts to reduce this major threat to birds.