These include Peregrine Falcon, American Kestrel, and even Sharp-shinned and Cooper's Hawk. Merlins, as falcons, have sharply pointed wings, unlike the broad rounded wings of Cooper's and Sharp-shinned Hawks. American Kestrels have a rusty back and tail. They also have a bold whisker mark, and a second mark down from they ear. They are much more heavily streaked on the underparts.
Merlins have only a faint whisker mark.
Merlins do not hover as kestrels. It's always difficult deciding on the size of a lone bird flying in the sky. Thus, Merlins may easily be confused with the larger Peregrine Falcon. Peregrines also have races which mimic the coloration of Merlins - especially the dark coastal races. Peregrines show a strong solid helmet which comes down to form a pointed cheek patch.