we have had a cardinal beating itself silly on ours. This is the second year in a row, last year it was a male to the front mirror and this year it's a female to the dining window.
Last year I covered the mirror with a trash bag taped to it and that calmed him down some, still would hit the bedroom window occasionaly though.
I'll be glad when their "courtship" ends and they settle down to being "parents".
