I've been meaning to post this for a while now, and want to share an unpublished tidbit that might be interesting to hawk enthusiasts (especially hawk banders). Of course, understanding molt sequence and feather patterns is important in ageing birds, and a while back, I mentioned the tail pattern quirks some raptors show that could be confusing in a few posts.
Anyway, on most raptor species, the innermost secondary (one right next to the body) is slightly paler than the rest of the secondaries of the same molt cycle (retained feathers of course can be quite pale). If you look at these images (an adult male Northern Harrier and several adult Red-tailed Hawks), you will see this, but don't mistake it for retained feathers from the previous molt. It is just a natural occurrence in raptors likely due being more exposed to the sun, and because the inner secondaries are replaced early in the wing molt. I show mostly 2nd-year birds here, so that we are sure the innermost secondary with an adult pattern are fresh feathers and not retained from the previous molt. So, just something to remember when you are looking at photos or have a bird in-hand...hope this is helpful.