1. Using the graphic in this lecture, I'd say that besides deer, crows prey on baby birds the least and that biggest predators of baby birds are snakes in the south and squirrels and chipmunks in the north.
2. I've been to 47 states and 5 provinces and have seen American Crows in all those places.
3. We don't have ravens where we live now but we see them often when I visit family in the Pacific Northwest. The size and beaks help me tell them apart. I've not observed enough raven behavior enough to distinguish them by that trait.
4. Our flock of chickens benefit from having crows around because not only do they sound the alarm when they see hawks (primarily Red-tailed and Red-shouldered, but often also Coopers, in our area), they also mob them, giving the chickens time to run and hide.