Ergodic properties of Gibbs measures on nilpotent covers

Let M = (M) over bar/Gamma be a closed negatively curved manifold with universal covering M and fundamental group Gamma. Every Gibbs equilibrium state v of a Holder continuous function on the unit tangent bundle (TM)-M-1 of M projects to a Gamma-invariant ergodic measure class mc(v(+)) on the ideal boundary partial derivative(M) over tilde of (M) over tilde. We show that this measure class is also ergodic under the action of any normal subgroup Gamma' of Gamma for which the factor group Gamma/Gamma' is nilpotent.