The harpies you've killed mentioned their matron, Serena Bloodfeather. Judging from their vile hatred toward the Horde, she must hold some kind of deep-seated grudge. You decide to hunt down Serena Bloodfeather and end matters once and for all.
HA! Well done, <name>! Very well done... I wasn't sure you were up to the task, but you've proved yourself quite the cut-throat. Thank you again for helping us suppress the Harpy epidemic. Here is your reward, use it well.