Bird mortality from window collisions is predicted and expected to occur wherever birds and windows coexist (Klem et al. 2009). Birds behave as if windows are invisible and they cannot recognize clear or reflective panes of glass as barriers to be avoided (Klem et al. 2004). This source of mortality is suspected to represent the largest source of avian mortality worldwide behind habitat destruction and predation by domestic cats (Klem 2009).

The first attempt to quantify bird mortality from window collisions was Klem (1990). His results were extrapolated to 1 – 10 birds killed per building and up to 1 billion total birds killed each year in the United States. 

Machtans et al. (2013) analyzed existing data and estimated 25 million birds are killed in Canada each year from window collisions. Residential homes are estimated to cause 90% of building-related morality, directly related to their large number compared to other building classes.

The University of Alberta’s Birds and Windows Project was designed to use citizen science and active participation to provide the most accurate estimate of bird window collisions at residential homes in Canada. Please contact us if you would like more information.


Klem D. 1990. Collisions between birds and windows – mortality and prevention. Journal of Field Ornithology 61(1):120-128.
Klem D. 2009. Preventing bird-window collisions. Wilson Journal of Ornithology 121(2):314-321.
Klem D, Keck DC, Marty KL, Ball AJM, Niciu EE, Platt CT. 2004. Effects of window angling, feeder placement, and scavengers on avian mortality at plate glass. Wilson Bulletin 116(1):69-73.
Klem D, Farmer CJ, Delacretaz N, Gelb Y, Saenger PG. 2009. Architectural and landscape risk factors associated with bird-glass collisions in an urban environment. Wilson Journal of Ornithology 121(1):126-134.
Machtans CS, Wedeles CHR, Bayne EM. 2012. A first estimate for Canada of the number of birds killed by colliding with buildings. Manuscript submitted for publication.