Looking at molt patterns on birds in flight can be very difficult to impossible to assess correctly (even photos can be misleading). I just spent the last 2 weeks looking at Bald Eagles on migration (among other birds), and discussing how to age them in flight with a number of birders. Let me say this up front -- knowing what to look for from each angle is far more important than anything else when ageing Bald and Golden Eagles!!!!! If you learn anything from this blog, I hope it is that. Not only that, but knowing what can't be done in the field is equally important. For example, an all dark Golden Eagle gliding overhead with its tail folded (closed) CANNOT BE AGED with certainty. Look at "Hawks From Every Angle" pg. 123 Figure GE 04. There is a reason I put that plate in the book, it is to show that the tail needs to be fully fanned to see if there are white based feathers or not. Golden Eagles with 1 to 4 retained sub-adult tail feathers are almost always impossible to age from below when the tail is folded. It is simply not possible to see the retained white-based feathers! So, uniformly dark Golden Eagles in these situations are "unknown age." Its silly to fret over saying "I don't know", it only means that you are aware or have the expertise to know better. Now, yes…it is possible to age all-dark Golden Eagles from below in some cases, especially if they are the rare-plumaged all-dark juvenile.
I got off subject a bit, but my original thought is this: look at the Bald Eagle image below ("click" to enlarge). It is the same bird in 2 different poses. In the first image, you cannot see that there are shorter inner secondaries. But in the second image with the wings spread, it is obvious. I bet some people would call the first image a juvenile based on the dark belly and what appears to be a lack of molt, but we know it to be a 2nd-year based on the image on the right. My point being, know what to look for in the field when molt is not visible, especially on birds such as this when the plumage is not the norm (and yes, brown-bellied 2nd-years are uncommon, but do fly by). Most times, 2nd-year birds are the whitest-bellied of all the ages (and that is important to know in itself). Note the very faded belly with minimal white patches, white on the crown, a yellow base to the bill, and the dark tips to the inner primaries. Hey, if you can see molt in the wings or tail great, but if not….learn what to look for otherwise. Learning the applicable field marks on flighted birds is one of the keys to accurate ID!