  Around one second after the big bang, neutrino decoupling and $e^+$-$e^-$
annihilation distort the Fermi-Dirac spectrum of neutrino energies. Assuming
neutrinos have masses and can mix, we compute the distortions using
nonequilibrium thermodynamics and the Boltzmann equation. The flavor behavior
of neutrinos is studied during and following the generation of the distortion.
