  We report results of 6.7 GHz methanol maser monitoring of 139 star-forming
sites with the Torun 32 m radio telescope from June 2009 to February 2013. The
targets were observed at least once a month, with higher cadences of 2-4
measurements per week for circumpolar objects. Nearly 80 percent of the sources
display variability greater than 10 per cent on a time-scale between a week and
a few years but about three quarters of the sample have only 1-3 spectral
features which vary significantly. Irregular intensity fluctuation is the
dominant type of variability and only nine objects show evidence for cyclic
variations with periods of 120 to 416 d. Synchronised and anti-correlated
variations of maser features are detected in four sources with a disc-like
morphology. Rapid and high amplitude bursts of individual features are seen on
3-5 occasions in five sources. Long (>50 d to 20 months) lasting bursts are
observed mostly for individual or groups of features in 19 sources and only one
source experienced a remarkable global flare. A few flaring features display a
strong anti-correlation between intensity and line-width that is expected for
unsaturated amplification. There is a weak anti-correlation between the maser
feature luminosity and variability measure, i.e. maser features with low
luminosity tend to be more variable than those with high luminosity. The
analysis of the spectral energy distribution and continuum radio emission
reveals that the variability of the maser features increases when the
bolometric luminosity and Lyman flux of the exciting object decreases. Our
results support the concept of a major role for infrared pumping photons in
triggering outburst activity of maser emission.
