Originally Posted by Zaydos
Well I do have an idea where to put them (actually two places though one is on the eastern half of the map which I've never finished) but as for CR, I'd say CR 15 with Treasure: None would balance the item.
The logic being a CR 15 creature ought to give 22,000 GP of treasure, this way the item is now a use-activated item of something between Resurrection and Raise Dead (Resurrection would place it at just under 15,000 GP) or can be used to replace the material component of True Resurrection as an alternative option should you find a priest with such capabilities.
Of course, I'll probably make them even stronger to put them more on the scale of legendary creatures aiming towards CR 18ish, or with "Young" "Adult" and "Elder" categories and give them either other harvest-able organs (I like power components) or something. Make them war with the white and silver dragons that share their mountain peak homes.
I'd suggest the "Young", "Adult", and "Elder" categories personally.
I like them as Good/Neutral aligned because it presents a fairly significant moral dilemma for non-Evil players, especially Good-aligned players, to harvest Eagle stones from them, or perhaps even use them. "You can revive the slain innocent child, but you must slay a noble creature to do so" sort of thing.
A few other things sometimes attributed to Thunderbirds in mythology:
Massive size: I've heard parallels with the Roc for how amazingly big some can get ('blot out the sun'). Certainly something for the "Elder" category I think.
Human form: I came across a myth once that said Thunderbirds could take human form by shedding their feathers like a blanket. The downside of course being that they couldn't return to their birth form if it was stolen from them. I doubt that it could turn anyone other than the true owner into a Thunderbird, but it's another potential magic item. The biggest trick is that it has
to be stolen while the thunderbird is in human form since no one except the thunderbird themselves can 'remove' it in this fashion.