I have heard the explanation before of meat color variation based on amount of usage. The mention of it again forced me to do al ittle research as to WHY this is. Not being a biologist myself I had to think back to HS bio - here's what I was able to find FYI.
"Because myoglobin is used for oxygen storage, the amount of usage a muscle gets will determine the amount of myoglobin in the fibers. Muscles are composed of two different types of muscle fibers which vary in proportions between muscles. Fast-twitch, or white fibers, have a low myoglobin content, since they depend on anaerobic Glycolysis for energy production. Slow-twitch, or red fibers have a high myoglobin content, since they depend on the aerobic TCA or Krebs' Cycle for energy production. So dark meat color is a result of a relatively high concentration of slow-twitch fibers in the muscle of the animal. Muscles that are used constantly, like legs, have more slow-twitch fibers, so their meat is much darker, than muscles that get little use, like the wings on a chicken. In fact, if you look at the wings of a birds that flies alot, like a duck, the meat is very red."
For the complete text, see http://www.madsci.org/posts/archives/ma ... .Gb.r.html
Having only hunted Spruce and Ruffed myself I can't speak to the habits of sharptails, Ptarmigan but this does provide a good explanation.