As requested by Mike Marsh, the bird in question is a juvenile male Cooper's Hawk. His e-mail summarizes most of the field marks quite well.
Here are some other features to cue from in Nancy's excellent pictures.

1. The size of the eye. Although Cooper's Hawks have a much larger head than a female Sharpy, the eyes of both species are the same size. So the eye will look much larger in proportion to a Sharpy head. The eye in this picture is relatively small. I think this is the most useful feature in the back shot.
2. The bird is extending its crest slightly for some reason. Cooper's have a crest, Sharpies never do.
3. As Gene Hunn pointed out earlier, the outer tail feathers (guard feathers) appear to be shorter than the central tail feathers. In Cooper's the guards are roughly 12 MM shorter. In Sharpies they are the same length as the central tail feathers, hence the famous, or perhaps infamous, "square tail". You have to look closely to see this in the photo.
4. The head profiles between the two species are entirely different. Sharpies have a slight step or forehead at the confluence of the cere and forehead. Cooper's have this really elegant smooth line without a forehead.
5. In the side shot, you can see the typical classic "extended tear drop" breast feathers on a white breast. Sharpies usually have a broader "paint-brush" streaking in most, but not all, cases.
And thank you for the kind words Mike, but I have to say that most hawk banders out there could have answered this one too. So, in my view, one of the very best ways to learn your accipiters is in hand at your friendly neighborhood fall hawk banding station where literally hundreds of these birds are tagged and released each year. Most stations do accept volunteers to help band.
Finally, as a thank you to those taking the time to read through all this, I just saw an immature brown female Gyrfalcon this afternoon with a full crop sitting in a tree here on the Samish Flats just east of Chuckanut Drive. This is possibly the bird Dennis Paulson and crew had earlier in the winter down on Sullivan Road.
I hope we keep on getting these remarkable photos to look at on Tweeters. There are so many extraordinary shots by some really superb photographers these days. Thanks for your efforts.

Bud Anderson
Falcon Research Group
Box 248
Bow, WA 98232 USA
(360) 757-1911
bud at frg.org