
Originally Posted by
GrahamH
The imprint is dirt and dust (possibly other stuff) from the bird.
If you can, use an off camera flash (or other light source) to backlight the image, then shoot the image.
If you use a lamp or other low powered light, place your camera on a tripod and use a suitable long shutter speed.
Leave the light source out of camera view (or flag it off - that is, place something between it and the camera, so as to eliminate flare) somewhere behind (not in direct line with) the imprint. For example, if the imprint is 6' above ground level, place the light source several feet behind the window below the imprint by at least a couple of feet. Place the camera on a tripod and set fairly close to level (if possible) to the imprint (reduces DoF requirements). Take picture. Don't be surprised if ALL the other dirt on the window is revealed, you may need to clean around the imprint and retake (quicker than photoshopping it all out).
The backlighting technique (with flash) works great when capturing smoke trails which is similar in this respect to the imprint.
Hope that helps.
Graham