Daniel Klem is a leading expert on collisions of birds with windows.
He published "Bird-Window Collisions" in 1989 in Wilson Bulletin 101(4):606-620. Here is the URL: www.birdscreen.com/PDF/KlemCollisions1989.pdf. All birds are vulnerable, because they fail to recognize glass as a barrier. Birds that fly into windows are not necessarily unhealthy or otherwise impaired specimens.