They certainly can be! They're fairly big -- though nowhere near as big as a great-horned owl -- they're smart, and they're good at teamwork. I watched them chase that poor owl like a relay race in teams of 2-4 crows. Crows are known for mobbing interlopers and can often drive away much bigger animals.

I felt so sorry for the owl. But I guess they all considered my yard good enough to fight over for months on end. I have ~2 acres of laissez-faire permaculture amidst monocrop corn or bean fields, and big birds like to perch in the big old trees. Occasionally I see a redtail hawk. Last spring, a pair of bald eagles found it a romantic carpet for their courtship dance, that was exciting. :D