At low wind speeds the shortest capillary waves appear to be generated hydrodynamically and not by the wind. This phenomenon is investigated using a Hamiltonian representation of the surface wave dynamics. A perturbation technique of Kolmogorov is used to transform away non-resonant, nonlinear interactions. Resonant interactions are treated by the Hasselmann transport equation, applied to the transformed variables. Calculated spectra show reasonable agreement with the observations of Jähne & Riemer (1990).