Each year red-tails undergo a complete molting of their feathers. The first molt (at one year of age that results in the setting of the adult plumage)
occurs in the early spring. Subsequent molts, though, occur at less precisely timed intervals. They may be observed in the spring, in the
summer or even extending into the early winter. The molt progresses in a well defined, orderly succession and never denudes
or disables the bird.

http://www.psu.edu/dept/nkbiology/naturetrail/speciespages/redtailhawk.htm