Blood shed in Ogden Garden

We have been watching a crow on the bird bath in the garden. Initially we thought it was having a drink but then it was apparent it was eating some sort of meat. We went out to investigate but it flew away with remnants of what it was eating leaving only a pool of blood. It returned  a short time later and was repeating its behaviour. We went out again and this time it flew away leaving its meal behind. Its part eaten meal was a frog. Frogs do not live in our bird bath! Has anybody witnessed this sort of behaviour before?