  In the redshift range z = 0-1, the gamma ray burst (GRB) redshift
distribution should increase rapidly because of increasing differential volume
sizes and strong evolution in the star formation rate. This feature is not
observed in the Swift redshift distribution and to account for this
discrepancy, a dominant bias, independent of the Swift sensitivity, is
required. Furthermore, despite rapid localization, about 40-50% of Swift and
pre-Swift GRBs do not have a measured redshift. We employ a heuristic technique
to extract this redshift bias using 66 GRBs localized by Swift with redshifts
determined from absorption or emission spectroscopy. For the Swift and
HETE+BeppoSAX redshift distributions, the best model fit to the bias in z < 1
implies that if GRB rate evolution follows the SFR, the bias cancels this rate
increase. We find that the same bias is affecting both Swift and HETE+BeppoSAX
measurements similarly in z < 1. Using a bias model constrained at a 98% KS
probability, we find that 72% of GRBs in z < 2 will not have measurable
redshifts and about 55% in z > 2. To achieve this high KS probability requires
increasing the GRB rate density in small z compared to the high-z rate. This
provides further evidence for a low-luminosity population of GRBs that are
observed in only a small volume because of their faintness.
