During nesting season, the Bluebirds are very territorial.  I've read that Eastern Bluebird nest boxes need to be at least 300 feet apart and my Mr. Bluebird checks every box on the property to ensure his territory is secure!  Sadly, he can't distinguish between an actual intruder and his own reflection.  In between checking all the houses on the property, he's hard at work attacking EVERY reflective surface on the property.  In the images, below, Mr. Bluebird chases his reflection in the windows at the front of my house in the morning -- and at the back of my house in the afternoon.  My den sits at the back of the house, so I listen to "clunk, clunk, clunk" all afternoon as the reflective intruder is attacked.

Mr. Bluebird has been attacking his reflection in the windows of this house for years.  There are times I contemplate whether he is actually following me around the house -- knocking on the windows to let me know when he is out of mealworms.  I wouldn't be surprised!

This year, however, Mr. Bluebird has taken to also attacking his reflection in the glass of the video camera located in front of the nesting box he and his mate have chosen.  I feel for him as he has been after that Bluebird in the camera ALL DAY LONG for weeks.