I have seen a huge amount of crows come together specifically to attack and murder a bird of prey as well as take down larger prey.

Highly efficient highly intelligent highly mobile.....

And on top of it extremely hard to kill. I should know out of all of the birds I have shot I have only gotten one crow.

Actually I shot one and looked and was amazed to discover that with the small body and large amount of feathers most of my shots that I thought hit would just pass through the feather layer leaving the crow completely unharmed. All those laws of average eventually caught up to one crow though.