Many birds are very protective of the area they are living in. When a bird sees his/her reflection in your window he/she thinks it is an invading bird to that territory, so it attacks. Solutions can include hanging a few strips of ribbon outside the window,this causes the image to be distorted. Also you can hang something inside the window, such as a square of reynolds wrap. I have a dove stained glass art work hanging in one window and it does get attached from time to time. Hope this helps.