GR translation: feathering out like trailing edges of hawks’ wings

[Mike Jing]

V464.20-24, P472.4-8   “Beg pardon?” Slothrop looks around and finds Miklos
Thanatz, full beard, eyebrows feathering out like trailing edges of hawks’
wings, drinking absinthe out of a souvenir stein on which, in colors made
ghastly by the carnival lights on deck, bony and giggling Death is about to
surprise two lovers in bed.

What does "feathering out" mean here?