I've seen Pike fish rise out of the water and eat moorhens. They also eat ducklings. It seems a bit backwards, doesn't it? A fish eating a bird. Pike can grow quite big and they make quite the splash when they snatch something from the surface of the water.
One time I saw a rabbit on the pavement on the verge of death. You could tell it was dying and that there was no saving it. When an animal is that close to death, it can be unnerving how still they are. The rabbit had this haunting expression that's difficult to describe.
I have seen rival birds throw out the eggs of other birds. Cuckoo birds are rather infamous for this behaviour. They steal nests and throw out any eggs that were in the nest.
Swans are by far cruel and stupid. Two male swans had to be separated recently because they kept trying to kill each other. Swans are territorial and stubborn. One time I tried to leave my house and a bunch of swans tried to attack me. Made me late for work. 
I once witnessed baby chickens eat their half developed sibling. I was around nine or ten. Uh, not so fun fact but you can buy spray for chickens to make them less likely to engage in cannibalism. Whether or not it works is another question.
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