Red eyed vireo

I was minding my own business, working on the computer when a series of bangs on the window made me jump.  A greenish bird with its mouth open perched on a nearby perennial poppy and then hit the window again.  

This behaviour is common with male birds during breeding season.  They see their own reflection and figure it's a competing male, so they fly at it to scare it away.  Imagine their puzzlement when they hit a solid invisible wall of glass!  

To prevent this behaviour, you can take a bar of soap and scribble on the outside of the window.  That will break up the reflection and stop the bird's confusion.

I just ran at the window with my arms flopping and yelled at it.  That worked too.