Similar to the Caragor meat, you will find these nests hanging around. Simply using your Wraith's bow, knock an arrow into the hive to knock it down onto any Uruks below. Or, if it is hanging over a set of lines you can walk across, you can press
to Terrorize any enemies that lie below. You can always Attract the enemies to hang around below you, or you can just get them as they patrol/pass under. Simply have to get about twenty to flee.
You might want to try going in to the Strongholds for large amounts of Uruk.