Do Lighting Conditions Influence Bird–window Collisions?

Bird-window collisions account for approximately one billion bird deaths annually in North America. Highly reflective or mirrored glass is associated with increased collision risk, but little is known about whether the reflection caused by differential lighting of otherwise clear glass influences the risk of window collisions. We aimed to determine whether reflection from a clear window influences daytime collision risk by manipulating the lighting conditions on exterior and interior window surfaces. In a flight tunnel, we flew domesticated Zebra Finches (Taeniopygia guttata) toward windows manipulated to be of higher or lower reflection and recorded collision risk and flight velocity using three-dimensional videography. We predicted that the risk of collision would be greater when windows were manipulated to be more reflective. We found no support for this prediction. In contrast, we found that collision risk decreased in the presence of a stronger reflection during bright, midday exterior-lighting conditions. We suggest that the influence of window reflection on daytime window collisions is more complex than often assumed and might involve previously unaccounted properties of light, such as the polarity of light. Lastly, we recommend directions for future collision research and non-invasive mitigation strategies which involve the manipulation of interior lighting throughout the day.