Rocs are colossal avians, the size of mountains, who lair in high mountains and who wing over coast and sea, preying on whales, elephants, and dire animals, usually dropping the animal from a great height so that it splatters on the ground. They also menace ships in the region, and are fierce defenders of their nests and eggs (which, being immense, are often sought out as food sources).
Rocs attack prey or annoyances with claws and beak. Their favorite hunting tactic is to grab prey with their talons, soar up into the air, and then drop them from a great height. They have also been known to employ boulders to pulverize ships at sea.
Roc eggs, though dangerous to harvest, are huge and flavorful, and can provision an entire ship. Roc feathers are also quite valuable, as they are light, but durable, and immense to boot. Rocs can be trained like any other animal, but doing so is quite an endeavor for anything of human size -- simply keeping the creature in a nest and giving it enough food is a daunting challenge.
Roc feathers are used in wings of flying and brooms of flying. They can also be used to manufacture feather tokens.
Rocs are often followed by avian scavengers, such as eagles and buzzards, hoping to grab the choicest bits of a roc's prey as they splatter on the ground. Rocs themselves ignore these scavengers, though aren't above snapping up a few in their beaks if they get in the way.
Rocs are occasionally domesticated by storm giants and cloud giants, who train them as guardians and mounts. Rocs are majestic natural creatures, and quite rare, so their existence is of special interest to many druids.
Rocs prey mostly on large mammals, but are also fond of ankheg, purple worm, and harpy. Dwarves have a fairly adversarial relationship with rocs as well, considering them troublesome menaces -- most mountain dwarf communities crack roc eggs on sight, and disrupt the nests, while taking refuge from retribution underground.
The Chaos Roc is a mightier, magical roc, with the power to emit a blast of prismatic light from its eyes (similar to the prismatic spray spell), and a tendency to gulp down live prey on the wing, rather than dropping it. Chaos rocs are intelligent, but highly unpredictable and quite disdainful of the company of others.
Chaos Roc: 3e Monster Manual II (2002)