  We present the results of a search for bright (-22.7 < M_UV < -20.5)
Lyman-break galaxies at z ~ 6 within a total of 1.65 square degrees of imaging
in the UltraVISTA/COSMOS and UKIDSS UDS/SXDS fields. The deep near-infrared
imaging available in the two independent fields, in addition to deep optical
(including z'-band) data, enables the sample of z ~ 6 star-forming galaxies to
be securely detected long-ward of the break (in contrast to several previous
studies). We show that the expected contamination rate of our initial sample by
cool galactic brown dwarfs is < 3 per cent and demonstrate that they can be
effectively removed by fitting brown dwarf spectral templates to the
photometry. At z ~ 6 the galaxy surface density in the UltraVISTA field exceeds
that in the UDS by a factor of ~ 1.8, indicating strong cosmic variance even
between degree-scale fields at z > 5. We calculate the bright end of the
rest-frame Ultra-Violet (UV) luminosity function (LF) at z ~ 6. The galaxy
number counts are a factor of ~1.7 lower than predicted by the recent LF
determination by Bouwens et al.. In comparison to other smaller area studies,
we find an evolution in the characteristic magnitude between z ~ 5 and z ~ 7 of
dM* ~ 0.4 mag, and show that a double power-law or a Schechter function can
equally well describe the LF at z = 6. Furthermore, the bright-end of the LF
appears to steepen from z ~ 7 to z ~ 5, which could indicate the onset of mass
quenching or the rise of dust obscuration, a conclusion supported by comparing
the observed LFs to a range of theoretical model predictions.
