The reason I ask this question is that one side of our lab building is attractive as a site for nesting house martins, Delichon urbicum, who place their nests close under the hanging eaves but in almost all cases immediately above a window. Ours is not the only building in the village where they build nests over windows and they also do it on some domestic premises. This placement of nest was something of a puzzle because we thought the birds would run a high risk of WBCs or could be disturbed in their flight paths by either the reflections or movements of people within. However, one warm summer all the windows were opened and the birds seemed a bit agitated, possibly because the casement windows looked like obstructions, but another issue proved to be the problem when we found the birds were having difficulty judging their approach and sometimes came in through the open window. So our conclusion from this was that the birds benefit from the windows by being able to see their reflections on the approach and thereby judge their approach path in much the same way as a pilot would use lights and other landing aids to judge the approach to an airstrip. I note that in your list of birds chimney swifts did not appear to be major casualties, so could it be that in that in the Hirundinidae and Apodidae, nearly all of whom nest in similar circumstances, the birds either instinctively or have learned to use reflections and shadows against walls or windows as flight aids whereas other birds cannot utilize these visual cues, and are possibly more likely to mistake them for competitor individuals of their own species?