why birds fly into windows

Chris Leahy of Mass Audubon explains male birds become territorial, and by singing they advertise they want to mate. But that singing also tells other birds to stay out of his space, said Leahy.
When they see themselves in a window something happens. “They don’t perceive that this is a reflection. They think it’s a rival, and they will attack this reflection,” said Leahy.
It should end when breeding season is over, but it will start up again in late summer when the second breeding season begins.
Experts say birds probably won’t hurt themselves doing this.