I wanted to recycle an old post from Uthabirders in regards to juvenile Red-tailed Hawks with reddish tails. I show examples in my books, but I wanted to point out the 3 birds in the photo above ("click" to enlarge). Note the juvenile bird on the left (©Tim Avery), it has one tail feather (left-center) that is shorter and the black sub-terminal band is slightly broader and bolder. The original feather fell out for whatever reason prematurely and the feather you see is a replaced feather. This newer feather has a bit of red, but is very similar in pattern and color to the rest of the tail. Most likely, this feather came in shortly after the bird fledged.
The bird in the middle (©Vic Berardi) has a few reddish
feathers on the right-center of the tail. These reddish feathers have
replaced a few juvenile feathers that came out for whatever reason
(snared in a bush, etc.). However, these reddish feathers are the same
color and pattern as adult feathers, meaning they most likely grew in
well after fledging.
The bird on the right (©Jerry Liguori) has a complete
set of juvenile tail feathers that happen to be reddish similar to
those of adults. Juveniles of all races can show this, but it most
common on borealis (Eastern race - which this bird is), and to a lesser
extent Harlan’s and Western. This is the original set of feathers that came in
during the nestling stage. The bird is a juvenile since it shows pale
primary “panels”, a brown upperside with whitish mottling lacking any
rufous tones, a pale eye, and the tail has juvenile-like banding. The
underside photos confirm this as well.