Feath, Feather, Feathest

I saw a cartoon in the New Yorker with three different pictures of Jupiter with a caption under each, “Jupit”, “Jupiter”, “Jupitest”.  Which got me to thinking about a piece entitled “Feath, Feather, Feathest”.   Once in a while an aviary bird dies in mid-molt, when its feathers are still growing and the owner gives me the dead bird.  This just happened with a Scarlet Macaw.  Its full grown feather would have been 20 inches or so long.  But this partly grown feath is just 7 inches.  When I take a full grown tail and cut some flying macaws out, it will become feathest.