Action: Angle windows to reduce collisions by birds
A single randomised, replicated and controlled experiment in the USA found fewer birds collided with windows angled away from the vertical.
Supporting evidence from individual studies
A randomised, replicated and controlled experiment in 1991 in Pennsylvania, USA (Klem et al. 2004), found that a fewer birds collided with windows angled at 20o or 40o from the vertical (28% and 15% of 53 recorded collisions respectively) than with vertical windows (57% of collisions). Six plate glass, wooden framed windows (1.4 x 1.2 m, 1.2 m off the ground, 15-43 m apart) were used, between January and May, on the edge of deciduous woodland and farmland.