Sergei studied the skull all day, while Xi and I talked about the Birdheads. They feature in the legends of a tribe of nomads who, hundreds of years ago, lived near The Sick Land. At least, that's what their descendants claim; it's hard to verify. Xi wrote her Master's dissertation on the subject, so she's an expert. I'll summarise what she told me.
The nomads and the Birdheads inhabited roughly the same area, the nomads further away from The Sick Land, the Birdheads closer. The nomads called them 'Birdheads' because of their bony beaks. Otherwise, they didn't look like birds.
The nomads and the Birdheads interacted frequently, sometimes trading, sometimes warring. Many of the Birdheads were completely inhuman; others barely differed from the nomads. The more wild legends are about the more monstrous Birdheads.
In the end, the nomads left The Sick Land. According to their stories, the son of the chief started growing a beak.