  Small galaxies are thought to be the main contributors to the ionising budget
of the Universe before reionisation was complete. There have been a number of
numerical studies trying to quantify their ionising efficiency through the
escape fraction $f_{esc}$. While there is a clear trend that $f_{esc}$ is
higher for smaller haloes, there is a large scatter in the distribution of
$f_{esc}$ for a single halo mass. We propose that this is due to the intrinsic
burstiness of star formation in low mass galaxies. We performed high resolution
radiative hydrodynamics simulations with Ramses-RT to model the evolution of
three galaxies and their ionising efficiency. We found that the variability of
$f_{esc}$ follows that of the star formation rate. We then discuss the
consequences of this variability on the observability of such galaxies by JWST.
