These two species share white eye-rings, a brown above/white below color pattern, and the habit of skulking around the edge of the water by themselves.
An alternate (breeding) plumaged Spotted Sandpiper (Actitis macularia) is an unmistakable bird with its bright orange bill and bold black spots on the breast. However the bird in the photo is a basic plumaged Spotted Sandpiper. It can be separated from Solitary Sandpiper (Tringa solitaria) in the following ways:
--structure Solitary has longer legs and a longer neck
--back pattern Solitary is darker and has distinctive, small white spots
--leg color Solitary is dull yellow/green, Spotted is orange/yellow
--tail length Spotted has a long tail that extends well past the wing tips
--Solitary has a dark wash on the upper breast and neck
--Spotted has a white spur in front of the wing see photo
--Spotted has a faint black line running in front of and behind the eye see photo
Both species are relatively common migrants through Iowa although they are rarely found in large numbers due to their solitary nature. Spotted Sandpiper breeds in Iowa and could potentially be found nesting almost anywhere there is water.
Aaron Brees, Mystery Bird Quiz Editor
In these photos the solitary happens to have its neck scunched in while the spotted is fully exended.