The mean momentum flux from the atmosphere into the short gravity waves of a wave system is modulated by the longer-scale flow, and the work done by this modulated stress outweighs the energy transfer from long to short waves due to the radiation stress. The sum of the energy flux due to the modulated wind stress and the direct transfer from the wind according to Miles (1959) can supply 10% or more of the energy required for the growth of the dominant ocean wave in the Hasselmann's model.