As for other theropod dinosaurs, _Yanornis_ and _Confuciusornis_ have been
found with fish remains in their guts.
_Yanornis_ also shows evidence of 'diet-switching', with a change between
herbivory and piscivory - perhaps depending upon the season and/or
availability of food (Zhou et al,, 2004). Other theropods may have done the
same.
Z. Zhou, J. Clarke, F. Zhang and Wings, O. (2004). Gastroliths in
_Yanornis_: an indication of the earliest radical diet-switching and gizzard
plasticity in the lineage leading to living birds Naturwissenschaften 91:
571-574.