wheatear: [16] The wheatear is etymologically the ‘white-arsed’ bird. The word is a backformation from an earlier wheatears, a singular form which came to be regarded as plural. And wheatears in turn was an alteration (due no doubt to confusion with wheat) of *whiteeres, a compound formed from white and arse. Like the parallel French term culblanc, it alludes to the white feathers on the bird’s rump. => arse, white
wheatear (n.)
type of bird, 1590s, back-formation from white-ears, literally "white-arse" (see white + arse). So called for its color markings; compare French name for the bird, cul-blanc, literally "white rump."