He came to our place a couple of weeks ago and has spent several hours a day crashing into our windows--particularly one on the backside of the house. He never injures himself, only hitting the glass hard enough to make a little sound and scare his mirror image. In my past job as a nongame biologist for the Kansas Department of Wildlife and Parks, I often talked to folks who had similar issues with over-aggressive birds at their homes. Birds are territorial, even in winter. They come to your house, especially if you have feed for them, they see other competitors and want to chase them off. The competitor in the mirror just won't leave though. Every day he's still there, vying for the same space at the same place.
Crash is driven to keep trying to drive his false competitor away. Is it a problem? It can get old hearing and watching him continually crashing into a window. It does provide some entertainment and certainly some opportunity for close photography. But, feeling sorry for him, we closed a rag in the window and that seemed to put a stop to his silly, mis-directed behavior. Crash seemed to have gone on to other Don Quixote windows to fight.