Answer:

turtledove

Answer	Crossword Clue
TURTLEDOVE	Animal twofer?
TURTLEDOVE	This animal presumably spends its entire life in a shell ... correction: just the first part of its life
TURTLEDOVE	One of two in "The Twelve Days of Christmas"

TURTLEDOVE	One of two in a Christmas song
TURTLEDOVE	Darling
TURTLEDOVE	Bird between the partridge and French hen
TURTLEDOVE	Christmas gift of song
TURTLEDOVE	Cooing bird
TURTLEDOVE	Term of endearment
TURTLEDOVE	Soft-cooing bird
TURTLEDOVE	Affectionate person.

TURTLEDOVE	Sweetheart, to W. C. Fields
TURTLEDOVE	Christmas-song creature
TURTLEDOVE	Sweetheart
TURTLEDOVE	French hen preceder
TURTLEDOVE	Affectionate bird
TURTLEDOVE	Reptilian pacifist?
TURTLEDOVE	Bird in a holiday song