Certainly not the most impressive photo, as this was a distant bird, but this crop does showcase how well they blend in---even after I found the owl, it took me several minutes to re-find it, each time I decided to change a setting or reposition myself.
This was at the Sandy Ridge Reservation in Lorain County, Ohio, which is a terrific spot for raptors---and home to at least two GHO nests. (Many more raptor and owl shots at my flickr: http://www.flickr.com/photos/noahcomet/)
7D & 400 f/5.6. Handheld, f/7.1, 1/800 (about as slow as I ever dare to go when handholding that lens) ISO 2500. Manual mode; spot metering. The hardest part about this shot---aside from finding the owl---was hitting focus, since there were many many branches at various distances along the way. (This is why I ended up stopping down a little---I needed some focus forgiveness!)
Thanks for looking!
Great Horned Owl by Noah Comet, on Flickr