Evidence, consequences, and angle of strike of bird-window collisions

Abstract

We used direct observation to record what, if any, evidence was measurable when a bird hit the outside surface of plate glass during 18 field experiments to evaluate several products to deter window strikes. A total of 1,356 strikes were witnessed over 508 d and 1,202 h of observation; 678 (50%) left no evidence of a collision, and 190 (14%) resulted in an immediate fatality. For 10 experiments, 1,261 detailed individual flightpaths were drawn over 235 d and 799 h of observation; 916 (73%) were strikes and 22 (2%) of these resulted in an immediate fatality. We recorded 822 (90%) flightpaths that hit perpendicular or within 40° on either side of perpendicular to the glass surface; 94 (10%) hit with a glancing blow of greater than 40° on either side of perpendicular. Perpendicular strikes resulted in 689 (84%) of individuals flying off immediately with no sign of impairment, 113 (14%) stunned, and 20 (2%) fatalities. Glancing blow strikes resulted in 81 (86%) flying off with no sign of impairment, 11 (12%) stunned, and 2 (2%) fatalities. Considering that 50% of bird-window collisions leave no measurable evidence of a strike, and as much as 70% of stunned victims likely succumb from a collision, annual mortality may be minimally 1.28 billion-3.46 billion or as high as 1.92 billion-5.19 billion in the United States, with potentially billions more worldwide.