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.
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.