Cause I am short of time at the minute I have found a piece from an article on avian anatomy. Also a link to an article on Glass as a hazard for birds. The pics of the windows are so real that it is impossible for a bird not to think that they are flying into a nature scene.
"Since the neck forms an "S" curve, it protrudes forward in the front, above the level of the crop. Often, this may be mistaken for a tumor or abnormality in the neck, especially when the crop is empty and the bird is sitting comfortably. Because the neck has more vertebrae than a human's and mammal's, the avian neck is extremely flexible, mobile and strong. We've all seen how easily a owl can turn its head so much farther around than we can. When a bird is comfortably restrained by an avian vet, the head and/or neck is held. The neck is considered one of the strongest parts of a bird's body, and it is almost impossible to injure a bird by holding it by the neck (as long as the windpipe is not closed off), let alone break its neck, when it is properly restrained. Often, people think, when they pick up a limp, dead bird, that it must have broken its neck, because the neck is so limber. It rarely is the cause of death. Birds that fly into a window or other solid structure may die, often of a concussion or other trauma, but in all my years of practice, I have only seen two birds with fractures of the cervical vertebrae."
Reference: 2006 Margaret A. Wissman, D.V.M., D.A.B.V.P.