  Using SofI on the 3.5m New Technology Telescope, we have conducted an
extensive near-infrared monitoring survey of an unbiased sample of 69 brown
dwarfs spanning the L0 and T8 spectral range, with at least one example of each
spectral type. Each target was observed for a 2-4 hour period in the Js-band,
and the median photometric precision of the data is ~0.7%. A total of 14 brown
dwarfs were identified as variables with min-to-max amplitudes ranging from
1.7% to 10.8% over the observed duration. All variables satisfy a statistical
significance threshold with a p-value <5% based on comparison with the median
reference star light curve. Approximately half of the variables show sinusoidal
amplitude variations similar to 2M2139, and the remainder shows short timescale
evolving light curves similar to SIMP0136. The L/T transition has been
suggested to be a region of a higher degree of variability if patchy clouds are
present and this survey was designed to test the patchy cloud model with
photometric monitoring of both the L/T transition and non-transition brown
dwarfs. Considering the targets identified as variable with the same
statistical threshold, the measured variability frequency of 13^{+10}_{-4}% for
the L7 -- T4 transition region is indistinguishable from that of the earlier
spectral types (32^{+11}_{-8}%), the later spectral types (13^{+10}_{-4}%), or
the combination of all non-transition region brown dwarfs (21^{+7}_{-5}%). The
variables are not concentrated at the transition, in a specific colour, or with
binaries. We note that of the systems previously monitored for variability only
~60% maintained the state of variability (variable or constant), with the
remaining switching states. The 14 variables include nine newly identified
variables that will provide important systems for follow-up multi-wavelength
monitoring to further investigate brown dwarf atmosphere physics.
