I agree with Kelly that Roger Tory Peterson's Western Birds (3rd ed, paperback) mislabels the merlin subspecies -- the lightest should be Richardsons. I went back to my copy of his earlier book on Western Birds (the only date I could find was 1941) back when Merlins were called Pigeon Hawks, and his written descriptions of the subspecies agree with everyone else.

I'd also guess that I'm not sure that considering only the size and blue-gray back would distinguish a Merlin from a male, adult Sharp-shinned Hawk. Wheeler and Clark's Photographic Guide lists male sharp-shinned's as being 9-11 inches long, and weighing 3-4 ounces. They show a nice picture of the back view on page 34, photo SS05.

Wheeler and Clark show a soaring Taiga Merlin on page 137, photo M02, which looks pretty close to the same shade of gray as the sharp-shinned. They don't break out the Merlin's measurements by male/female, but Kate Davis, Falcons of North America (2008) lists a male Merlin as 9 to 11 inches long as well, and weighs 5.6 to 6 ounces. So if one were looking for the smaller of the two, the sharp-shinned actually weighs a good bit less.

I wouldn't be surprised that either would be described as swooping around agilely and with much speed, when chasing birds at a feeder. Wouldn't the best way to start off distinguishing a flying Merlin from a sharp-shinned be to focus on the shape of the wings, pointed for the Merlin and relatively shorter and more rounded for the sharp-shinned, and also consider the style of flight, more powerful and direct for the Merlin, compared to a sharp-shinned which is more likely to change directions and perhaps dive between or into bushes?

Eric Stahlfeld
Seattle, WA