From the Cornell Lab of Ornithology in the USA:
How Does a Crow Win Against a Raven? With a Little Help From Its Friends
[American] Crows may be smaller than Common Ravens, but when they join forces, they can stand up to their larger cousin. Over the years, eBirders have noted crows mobbing ravens more than 2,000 times, and researchers used that data to explore the birds’ rivalry. They discovered that crow mobs can be effective at driving away ravens. The crows’ social nature may work in their favor against the more solitary raven. Read the full story.