  We model the chemical evolution of six UFDs: Bootes I, Canes Venatici II,
Coma Berenices, Hercules, Leo IV and Ursa Major I, based on their recently
determined star formation histories. We show that two single-age bursts cannot
explain the observed [$\alpha$/Fe] vs [Fe/H] distribution in these galaxies and
that some self-enrichment is required within the first burst. An alternative
scenario is modelled, in which star formation is continuous except for short
interruptions when one or more supernovae temporarily blow the dense gas out
from the centre of the system. This model allows for self-enrichment and can
reproduce the chemical abundances of the UFDs in which the second burst is only
a trace population. We conclude that the most likely star formation history is
one or two extended periods of star formation, with the first burst lasting for
at least 100 Myr. As found in earlier work, the observed properties of UFDs can
be explained by formation at a low mass ($M_{\rm{vir}}\sim10^7$ M$_\odot$),
rather than being stripped remnants of much larger systems.
