We woke to another frosty morning yesterday, but it was not only the frost that had left its mark. As you can see, it looks as though a bird collided with the glass in our sliding door. I have only seen these imprints on a couple of previous occasions. One was on the window at a house in Swansea, and was clearly made by a large bird, perhaps an owl.
I was relieved that there was no other evidence of yesterday's accident, but I have no means of knowing if any real damage was done. Judging by the size of the imprint (something that the photo does not show well), I would guess that the bird was about pigeon-sized.
These imprints or images seem (in many cases) to be caused by 'powder down', which you can read about here - or, if your server allows access, here.
Richard Heeks began a Flickr group for sightings of bird imprints. He suggests that head-on images may be caused by birds who mistake their own reflection for a rival or enemy. I have certainly watched a Blue tit pecking away at its reflected self in my wing mirror.
Here is some sound advice from the RSPB: it looks as though we need to buy some hawk silhouettes.