  The HiRes and AGASA Collaborations have published contradictory energy
spectra for energies above the GZK cut-off. In this article, we investigate the
acceptance of fluorescence telescopes to different primary particles at the
highest energies. Using CORSIKA shower simulations without and with the new
pre-showering scheme, which allows photons to interact in the Earth magnetic
field, we estimate the aperture of the HiRes I telescope for gammas and protons
primaries. We calculate the dependence of the telescope sensitivity to primary
particle identity. We also investigate the possibility that systematic
differences in shower development for hadrons and gammas could mask or distort
vital features of the cosmic ray energy spectrum at energies above the
photo-pion production threshold. The impact of these effects on the true
acceptance of a fluorescence detector is analyzed in the context of top-down
production models.
