The Carolina Chickadee feeds in flocks. It's a very inquisitive bird; often being the first one to try out a new or re-located feeder. It almost always stays in the same flock (extended family?) from birth to death.
Actually, Robins' favorite food, the earthworm, isn't native to North America. Some portions of this continent don't even have ANY earthworms. To muddy the waters further, what's called a "robin" in England isn't the same bird. To help alleviate confusion the Robin here, which is a species of thrush, is called the American Robin.