I got an e-mail the other day about why some birds look like they have dark areas in the flight feathers such as the Cooper's Hawk below (top bird). The person who asked knew the answer but I thought I'd just show it anyway. Just so happens when birds are backlit the overlapping feathers can show up as dark areas, it has nothing to do with molt or anything else.
Also, light can effect other things too. Check out these 2 juvenile Red-tails…sometimes the translucent primary "windows" don't show up in flight. AND, sometimes adults show translucence for certain reasons, now those are tricky. If possible, always ID or age birds by more than a single trait alone, I can't stress that enough!!!!! Hope this helps.