Fall hawk migration is just about starting, in fact, I just saw a Swainson's Hawk and Kestrel moving south along the Wasatch Range on my hike yesterday. This means hawk migration counts and hawk banding projects will be in full swing soon and I wanted to relay this message to all the hawk banders out there. Hawk banders already know to look for retained feathers (or two generations of feathers) when they are determining the age of a raptor in hand, and on certain birds it is helpful to look at specific body feathers, such as upperwing coverts and rump feathers. But, remember to check out the underwing coverts too (and especially the underwing linings). Some raptors tend to replace the underwing coverts last, especially when molting from juvenile to adult plumage. Even well known birders have admitted to having difficulty determining juvenile from adult underwing coverts, but I urge bird banders to look for this in-hand. It's actually pretty obvious telling juvenile from adult underwing coverts on some species, especially adult male Harrier, Northern Goshawk, Mississippi Kite, Gray Hawk, Peregrine Falcon, and Gyrfalcon.
Check out these adult male Harriers (images #1-3, note the last in hand Harrier lacks retained feathers for comparison, so the age is simply "adult"), adult Goshawks (image #4), adult Peregrine (image #5), and adult male Hen Harriers (image #6), the difference in the 2 Hen Harriers is quite striking, the plumage is identical but the brown juvenile feathers of the bird on the right are extremely obvious since the underwings lack the spotted look of adult male Northern Harrier. Anyway, note the juvenile feathers (browner ones that differ in pattern) that stand out from the adult feathers on the second-year Northern Harriers and Goshawks. This is not obvious on Peregrines since the pattern is similar, so don't confuse retained adult feathers from juvenile feathers (note the retained juvenile flight feathers that confirm the age). It is much easier to tell retained juvenile from adult underwing coverts on Gyrfalcon than on Peregrine.
Remember to look for this next time you have an adult hawk in hand and can't find any signs of molt...you might find some retained juvenile feathers along the underwings.