A few discussions came up about hawk ID this past week. One in particular was ageing Bald Eagles to a specific year. Let me stress that LOOKING AT PHOTOS IS VERY DIFFERENT THAN LOOKING AT BIRDS IN THE FIELD! I have said this before, and it is extremely important to realize this in making accurate ID's. There are many traits that can be seen in close-up photos that are often 'invisible' in the field. So, knowing what to key in at first glance for birds in flight may be different than what to look for in a photo. Knowing this is half the trick!
Most birders know how to tell 1st-year (juvenile) Bald Eagles, but some 3rd and 4th-year Bald Eagles (and some 2nd-years) appear dark-bellied and dark headed in the sky and can be tricky to tell from 1st-years. So, what do you look for on dark-bellied Bald Eagles overhead? One of the neatest things to look for are the "translucent" tips of the inner primaries that appear as if 1st-year birds are missing feathers. Older birds may have white on the inner primaries, but the tips of the feather are black, so the wings lack the "missing feather" look of 1st-years.
Also, when 1st-year birds bank in the distance and reveal the upperside, the overall tone to the topside is helpful. Of course most 2nd and 3rd-year birds have a white patch on the upper back, but many 3rd and especially 4th-year birds lack an obvious white patch. However, they also lack the two-toned upperside (evenly brown upperwing coverts contrasting the blackish flight feathers - that becomes very obvious by winter) that 1st-year birds show, instead having a darker overall topside. 2nd, 3rd, and especially 4th-year Bald Eagles may show faint to obvious white on the top of the head, juveniles always show dark heads (but some have white on the throat).
Check out these 1st-year birds on top (and bottom right - upperside photo) compared to the 3rd-year birds on the bottom. And remember, of course no field mark is 100% accurate, there are exceptions to every rule!