Of several hypotheses proposed for the origin of feathers two predominate: feathers evolved for flight, or for thermal insulation. The argument is sometimes made that since wing feathers degenerate with flightlessness, their primary function is aerodynamic, supporting the flight hypothesis. Examination of the primary feathers of flightless carinates reveals little evidence of degeneration. Notwithstanding the impropriety of deducing original from present-day functions, feather structure in flightless carinates does not support one evolutionary hypothesis over any other.

Ratites have markedly different primary feathers from flightless carinates and this might be attributable to the longer time since the loss of flight.