Neutrino clustering and the Z-burst model

The possibility that the observed Ultra High Energy Cosmic Rays are generated by high energy neutrinos creating Z-bursts in resonant interactions with the background neutrinos has been proposed, but there are difficulties in generating enough events with reasonable incident neutrino fluxes. We point out that this difficulty is overcome if the background neutrinos have coalesced into neutrino clouds --- a possibility previously suggested by some of us in another context. The limitations that this mechanism for the generation of UHECRs places on the high energy neutrino flux, on the masses of the background neutrinos and the characteristics of the neutrino clouds are spelled out.