  We give a geometric description of the Poisson boundaries of certain
extensions of free and hyperbolic groups. In particular, we get a full
description of the Poisson boundaries of free-by-cyclic groups. We rely upon
the description of Poisson boundaries by means of a topological
compactification as developed by Kaimanovich. All the groups studied here share
the property of admitting a sufficiently complicated action on some real tree.
