Thunderbirds originally come from Native American mythology, sharing similar characteristics to Eurasia’s rocs/phoenixes/firebirds, albeit with a more stormy focus. The Bestiary 2 notes the complex and careful relationship local tribes often have with thunderbirds, seeing them as bringers of both life-giving rains and village-destroying hurricanes. In your campaign they might be a nice regional replacement for other large avians like rocs and giant owls, servants of storm deities, allies of the djinn (seeing as they speak Auran and are as intelligent as humans), enemies of dragons, or hazards of aerial travel. However you use them, they are quite literally forces of nature that bring storm and lightning in their wake.
The desert realm of Kalar does not get natural rainfall. The only way to bring the rain is for a brave to roust a thunderbird out of its nest, either by polite entreaty in Auran or by force, so that it calls a storm in its agitation. Few young men and women ever return from this endeavor, but the desperate tribes have no choice but to continue sending volunteers—or troublesome outsiders.
Thunderbirds and primitive kongamatos wage epic battles in the skies above the Dagger Veldt, the thunderbirds’ size and sonic thunderbolts evening the odds somewhat against the smaller but more powerful dragons. Unfortunately, the territorial flyers are quick to break off their squabbles and even unite against intruders like skyships and adventurers on flying carpets.
The war on the Northern Front is not going well. If the Flying Foxes can’t find some way to protect their biplanes from Lizenne’s indigenous thunderbirds, how can they hope to stand up against the lightning guns and salamander dirigibles of the Fosterling King’s air force?
—Pathfinder Bestiary 2 264
This isn’t the first time we’ve seen a “thunderbird” in The Daily Bestiary…although the last time it was really a lightning elemental.