  The time-dependent Mandel Q parameter, Q(T), provides a measure of photon
number variance for a light source as a function of integration time. Here, we
use Q(T) to characterise single photon emission from a quantum emitter in
hexagonal boron nitride (hBN). Under pulsed excitation a negative Q parameter
was measured, indicating photon antibunching at an integration time of 100 ns.
For larger integration times Q is positive and the photon statistics become
super-Poissonian, and we show by comparison with a Monte Carlo simulation for a
three-level emitter that this is consistent with the effect of a metastable
shelving state. Looking towards technological applications for hBN single
photon sources, we propose that Q(T) provides valuable information on the
intensity stability of single photon emission. This is useful in addition to
the commonly used $g^{(2)}({\tau})$ function for the complete characterisation
of a hBN emitter.
