  The cosmic ray spectrum has been shown to extend well beyond 10^{20}eV. With
nearly 20 events observed in the last 40 years, it is now established that
particles are accelerated or produced in the universe with energies near
10^{21}eV. In all production models neutrinos and photons are part of the
cosmic ray flux. In acceleration models (bottom-up models), they are produced
as secondaries of the possible interactions of the accelerated charged
particle; in direct production models (top-down models) they are a dominant
fraction of the decay chain. In addition, hadrons above the GZK threshold
energy will also produce, along their path in the Universe, neutrinos and
photons as secondaries of the pion photo-production processes. Therefore,
photons and neutrinos are very distinctive signatures of the nature and
distribution of the potential sources of ultra high energy cosmic rays. In the
following we describe the tau neutrino detection and identification
capabilities of the Auger observatory. We show that in the range
3x10^{17}-3x10^{20}eV the Auger effective apperture reaches a few tenths of
km^2.sr, making the observatory sensitive to fluxes as low as a few tau
neutrinos per km^2.sr.year. In the hypothesis of nu_mu <-->nu_tau oscillations
with full mixing, this sensitivity allows to probe the GZK cutoff as well as to
provide model independent constraints on the mechanisms of production of ultra
high energy cosmic rays.
