I've lived in my house for 3 years. We have one window in our bedroom that has a fly problem. Every year there are hordes of what look like standard house flies between the window and the screen. I kill the flies and get rid of all the dead ones but every year they come back. Right now there are about 10 live flies and probably about thirty dead ones laying on the window sill.

Any idea on why this keeps happening or how to get rid of them? This is the only window with this problem.