After all the time I've spent playing I have actually started reading the unit histories.
These shrills are said to be constructs made by Morrigan the Mad, in the laboratories of N'it'Vaganesh - his attempts to manufacture monsters to rival the giant spiders that the Titan Curgen kept at his beck and call. The Earth shrill derives its power from the Earth Shard.
Their mandibles are potent magical reagents; practicing alchemists and wizards will pay top gildar for them.
Because of this I think shrills should drop a mandible that can be equipped for +5% spell mastery and be worth 25(?) gildar. Don't get mad at me, blame the story guy I expect a Steam hotfix in my game directory tomorrow. Ok get to it chop, chop