Penguins may swim and feed in groups, but some may be solitary when diving for food. Emperor penguins have been observed feeding in groups with coordinated divingDuring the breeding season some species come ashore and nest in huge colonies called rookeries. Some rookeries include hundreds of thousands of penguins and cover hundreds of square kilometers. Most species of penguins build nests, but the nests may consist only of a pile of rocks or scrapings or hollows in the dirt. Most birds have hollow bones to make their bodies light enough to become air-borne. But the penguins' heavy, solid bones help them float lower in the water. Flying birds need a large wingspan to hold them in the air, but small wings work best for birds swimming through the water.