For about a month now, a blue tit has been flying up to a window and pecking at it. There are no visible insects on the glass and the bird always goes to the same small area. It will do this throughout the day. More recently a chaffinch has shown the same behaviour at a different , also insect free, window as well as the front door glass.
Has anyone else reported this type of behaviour and does anyone have any explanation to offer
Dave Light

It is possible that the birds are seeing their own reflection in the glass. Many animals, when presented with a mirror, are unable to comprehend that they are seeing themselves in the reflection and instead assume that the reflection is another individual. The pecking may be some kind of territorial display or other social behavior.