I've been having this problem recently and even found one that had died. I started looking on line and found, I hope, a cheap, easy to remove solution I'm trying. Just take a bar of bath soap and draw sqiggles or lattice designs on the inside of the glass. Has anyone else tried this?
I haven't tried the soap but anything that will signal to the birds that there isn't an open area they can fly to will keep them away from the window.