Harpies are weak against Grapeshot, Hybrid Oil and Aard.
Harpy nests are most often found atop high cliffs or rocky ravines. Sure signs of having strayed near one are crumbling human and animal remains, guano-streaked rocks and feathers littering the ground.
Harpies and shishigas hunt in flocks consisting of a handful to up to twenty individuals. Though rather cowardly and cautious, harpies fiercely defend their nests and will not hesitate to attack when outnumbering their foes.
During combat they use their ability to fly to dive swiftly to strike their victims one by one before soaring back up out of reach. They can kill with their wings or their sharp beak and talons. Once on the ground they move slowly and clumsily, and thus no longer present much of a threat.
Harpies are essentially wolves of the skies. They're mostly an annoyance, but still shouldn't be taken lightly if you're vastly outnumbered. They're quick and agile, and you'll need to bring them down to solid ground before you can easily finish them off.
Harpies are relatively harmless when they're floored, so either bring them down with your crossbow, or hit them with aard as they swoop. Either way, a few fast attacks while they're hopping about and they'll be history. The only real danger here is that you need to be careful not to be overrun from different directions when fighting whole flocks of the things, so be sure to keep an eye on your minimap to check the position of each enemy.