  The spectrum of collective fermionic excitations in a finite temperature
QED_{3+1} is studied in different regimes. It is shown that within the standard
perturbation approach the one-loop dispersion equation, besides the ordinary
one-particle excitation, has four new solutions. The additional excitations are
gauge-dependent and two of them have nonphysical signs of residues in the
propagator poles. The temperature evolution of the solutions is investigated
and it is shown that the use of effective propagators leaves no more than one
additional mode which becomes propagating at $T \bolpor 10M$, when the gauge
invariance is restored. The other three modes, including those with nonphysical
residues in the propagator poles, are always strongly damped, thus the thermal
effects do not produce pathologies in QED_{3+1}.
