  We systematically investigate temperature- and spectrally-dependent optical
reflectivity dynamics in AAs$_{2}$Fe$_{2}$, (A=Ba, Sr and Eu), iron-based
superconductors parent spin-density-wave (SDW) compounds. Two different
relaxation processes are identified. The behavior of the slower process, which
is strongly sensitive to the magneto-structural transition, is analyzed in the
framework of the relaxation-bottleneck model involving magnons. The results are
compared to recent time resolved angular photoemission results (TR-ARPES) and
possible alternative assignment of the slower relaxation to the
magneto-structural order parameter relaxation is discussed.
