Just a short note that I posted a while back on the UtahBirders blog. Yes, it is true that Black Merlins typically have less distinct whitish tail bands than the other races (Taiga and Prairie). However, many Black Merlins can have quite distinct tail bands, and Taiga (and a few Prairie) Merlins can have limited or no tail bands at all. I always say "almost no field mark is 100% reliable."

Check out the Taiga Merlins above ("click" to enlarge), the bird on the left (adult female) has a distinctly whitish-banded tail, the bird in the middle (juvenile) has limited tail bands, and the bird on the right (juvenile) lacks tail bands altogether. Note that many Merlins of all races do have a darker "tip."