Some of them do. Typically speaking larger birds will pick a turtle up and fly high into the sky where they then drop them to the ground where their shell breaks. This allows for the bird to get past...
Many things. Hawks, Owls, Kestrels, other birds of prey. Cats, snakes.There are many more. Anything that's a predator AND has the ability to move fast or hover in the air, will probably eat birds.