  Electromagnetic (and adiabatic) energy losses of pions and muons modify the
flavor ratio (measured at Earth) of neutrinos produced by pion decay in
astrophysical sources, $\Phi_{\nu_e}:\Phi_{\nu_\mu}:\Phi_{\nu_\tau}$, from
1:1:1 at low energy to 1:1.8:1.8 at high energy. The transition occurs over 1-2
decades of nuetrino energy, and is correlated with a modification of the
neutrino spectrum. For gamma-ray bursts, e.g., the transition is expected at
\~100 TeV, and may be detected by km-scale neutrino telescopes. Measurements of
the transition energy and energy-width will provide unique probes of the
physics of the sources. pion and muon energy losses also affect the ratio of
$\bar\nu_e$ flux to total neutrino flux, which may be measured at the
W-resonance (6.3 PeV): It is modified from 1/6 (1/15) at low energy to 1/9
(practically 0) at high energy for neutrinos produced in pp ($p\gamma$)
interactions.
