The poor creature is probably seeing what appears to be a 'rival' on its territory and attacking it.
Thousand of birds are killed every year flying into windows, the problem is finding a solution... Silhouettes of hawks have been fairly successful, but for a crow you might need quite a few, so the short term answer might be to hang something like a wind chime or those plastic streamers to distract it.
My cats love watching the crows attack the seagulls in the morning.
The two lazy morons sit on the windowsill (the side of the window with the central heating) and gaze at the scene. I think that the noise makes it more dramatic. Maybe this is TV for cats.
Some very interesting answers Folks but I think the best one is from Cetti. So I shall go to the local RSPB shop and hope they sell Silhouetted hawks(sounds interesting) and keep you posted. By the way Newtron what does LOL mean?
Hi moonshadow! Yes, the RSPB do sell them, but If you don't want to go to the bother of buying then I found this American site that obligingly gives a silhouette ready to print out.
Hi moonshadow, you could try some instant glass frosting, the stuff you just stick to the glass with water . You can pick it up at most hardwaer stores. It may distort the crows' reflection, therefore leaving you in peace. !
Thank you Flump1 for your suggestion but the window in question has a really lovely view of the countryside and I don't want to loose that. If anyone has similar problems or birds flying into the windows( which is another problem I have)the Birdsourse site Cetti recommended is brilliant.