1. While American Crows will sometimes eat songbird nestlings, the biggest predators of them are snakes and squirrels, accounting for over 50% of the predation. Even insects destroy more nestlings than crows.
2. Lots of places.
3. No.
4. Many other birds because of crows warning system, larger birds that prey on the crows, Great-horned Owls that use their nests, humans because they eat pest rodents.