The bulk update request #12709 is pending approval.
create alias brown_feather (0) -> brown_feathers (12395)
create alias green_feather (2) -> green_feathers (6316)
create alias grey_feather (15) -> grey_feathers (12141)
create alias orange_feather (0) -> orange_feathers (5849)
create alias pink_feather (12) -> pink_feathers (4773)
create alias purple_feather (18) -> purple_feathers (9918)
create alias red_feather (94) -> red_feathers (9820)
create alias tan_feather (0) -> tan_feathers (3139)
create alias teal_feather (0) -> teal_feathers (145)
create alias yellow_feather (25) -> yellow_feathers (18706)
create alias rainbow_feather (0) -> rainbow_feathers (163)
create alias two_tone_feather (0) -> two_tone_feathers (3428)
create alias multicolored_feather (0) -> multicolored_feathers (10052)
Reason: To be consistent with the black_feather/blue_feather/monotone_feather -> black_feathers/blue_feathers/monotone_feathers aliases.
Also relevant: topic #58526 for pending white_feather -> white_feathers alias.
However, there is one point to consider. Most of these <color>_feather tags are used for loose_feather/<color>_loose_feather posts.
I did consider either aliasing to <color>_loose_feather or disambiguating the whole set altogether, but I'm not sure if it would severely impact tagging due to autocomplete.