Big birds

When I sat down at my desk this morning I saw a heron in our garden being harrassed by a crow. I managed to get out my camera in time to take a (not-very-good) photo of the crow (in the centre of the photo) successfully chasing the heron away! I wonder why s/he did this. I expect the heron was looking for frogspawn or tadpoles, but they have already been eaten :-(