  We compute the luminosity function (LF) and the formation rate of long gamma
ray bursts (GRBs) by fitting the observed differential peak flux distribution
obtained by the BATSE experiment in two different scenarios: i) the GRB
luminosity evolves with redshift and ii) GRBs form preferentially in
low-metallicity environments. In both cases, model predictions are consistent
with the Swift number counts and with the number of detections at z>2.5 and
z>3.5. To discriminate between the two evolutionary scenarios, we compare the
model results with the number of luminous bursts (i.e. with isotropic peak
luminosity in excess of 10^53 erg s^-1) detected by Swift in its first three
years of mission. Our sample conservatively contains only bursts with good
redshift determination and measured peak energy. We find that pure luminosity
evolution models can account for the number of sure identifications. In the
case of a pure density evolution scenario, models with Z_th>0.3 Zsun are ruled
out with high confidence. For lower metallicity thresholds, the model results
are still statistically consistent with available lower limits. However, many
factors can increase the discrepancy between model results and data, indicating
that some luminosity evolution in the GRB LF may be needed also for such low
values of Z_th. Finally, using these new constraints, we derive robust upper
limits on the bright-end of the GRB LF, showing that this cannot be steeper
than ~2.6.

Astrophysics