Talking to the Harpies was simple for Kalona. They were tied, leaderless and had been oppressed for a while. They are also very stressed because they could never get a chance to breed more of their kind. The threat of their flock dying out was real and was constantly gnawing away at their minds.
Kalona being their savior and killer of the Flying Monkey already made her their new Matriarch in their eyes. When Kalona asked them to come back to her city, and breed freely; they wholeheartedly agreed.
Once they agreed, Kalona asked them, "Great, now I have a skill that can make you all more fertile and breed with other monsters easily. It is not as simple as that though. You'll become enthralled to me permanently as a tradeoff. I recommend accepting this if you plan on doing a lot breeding yourself."