grey heron

sew this grey heron stalking in the margins.i thought it had caught a fish but it had caught a bank vole instead.it then proceeded to eat it.
i have seen this heron attack a wagtail nest and eat the chicks.it also killed the mother and ate her
i did not have my camera with me that time.
i dont know why it behaves like this as there are plenty of fish in the river.