I have found mine eating some of my baby Koi fish. They love the eyeballs, I was not to happy with them about their choice of "snack" but they really enjoyed those Koi fish.
I fed mine goldfish by accident once!! I was empting a bucket. that I had used to hold my goldfish in while I was changing the water in their tank. Of course all the birds were running over to see what I was doing, and somehow two goldfish had escaped my net and remained in the bucket! Well once the birds saw them flopping on the ground they went nuts!! So now once in a while I buy extra feeder fish, just for them!