Argentavis

The bleached skull of an Argentavis pulled "from the mires of extinction" was used by the Faction Paradox member Father as his ceremonial mask. To match, his spouse, Mother, wore a mask made from the "weather-beaten" skull of a Haast's eagle. (PROSE: Going Once, Going Twice)

Behind the scenes
In the real world, the Giant Teratorn (Argentavis magnificens) is believed to have been among the largest flying birds in Earth's history, like the Haast's eagle.