Once again, I find myself turning to The Monsters Know blog, to think through the tactics of a specific monster my players are going to encounter more than once. The initial encounter during this last session was a nice appetizer and was pretty easy for them, as I expected. This is especially true with the mid-level (8th) of the PCs currently and their supporting numbers. However, what they have to expect (or perhaps not) is that this won't be the last time.
Reading through Harpy Tactics - The Monsters Know What They're Doing, I can see some nice suggestions on harpies using their luring song ability in some small scale sneak attack situations. I'm toying with the idea that the harpy reinforcements will follow the company and begin to set up traps and pick off some of the wayward squads in small raids. However, I think something the article writer missed and was pointed out in the comments, is that the song ability is an Emanation ability with a huge 300' range. This means it covers EVERYONE in that range. This is a huge globe. And when multiple harpies sing it overlaps, so anyone caught in multiple affects has to save for each one. Harpies aren't very intelligent, but perhaps there is a really BIG ambush that they might stumble into creating for the company. We'll see if that makes sense.
One bonus that I only just stumbled upon is specific lore surrounding harpies in Eberron. It turns out the Byeshk mountains, and not the Graywall mountains, are a natural place to find LOTS of harpies in organized groups. They sound like warring tribes. My plan was to have the group the PCs face be more monstrous, but keeping with the "monsters are people too" theme I keep toying with, I might have the PCs meet up with these Byeshk harpies at a future point. Again, we'll see.