  We show that the delay of structure formation can not fully account for the
reduction of electron optical depth from WMAP1 to WMAP3 when the radiative
transfer effects and feedback mechanisms are took into account in computing the
reionization history of the Universe. We also show that a PopIII stellar
cluster with a mass of 80Mo and a heavy Larson initial mass function has an
ionizing efficiency high enough to account for WMAP3 results, while in the case
of WMAP1, a higher stellar mass of 1000Mo was required.
