My window screens are on the inside of my casement windows. Any suggestions would be appreciated. Thousands of birds will pass a window and not give it a second look. Then, one bird discovers its own reflection and goes crazy. Presumably, the motivation is territorial, with the poor beast trying to chase away a perceived intruder.
Some birds become so animated and persistent that they injure themselves.
Any material that prevents or disrupts reflections from the glass will achieve what the bird cannot-remove the offending image. Newspapers or cardboard taped to the outside will work, but neither is very sightly. Mosquito netting and cheesecloth will work, too, and without obscuring your view out the window. I've had excellent results mixing a little detergent or baking soda in water. I apply the mixture with a spray bottle.