Vulture Mask

The vulture mask's general purpose is to protect the weak spot of the vulture: Its face. It can be obtained by hitting the vulture's mask with a spear right, or knocking it off with a powerful enough explosive. Be careful, however, because vultures that have been unmasked are twice as persistent and violent. After it's knocked off the vulture (or looted off its corpse), the mask can be worn in order to frighten off any animal scared of vultures, including most lizards and scavengers, but not squidcadas or other creatures not intimidated by vultures. Slugcat will wear the mask when not otherwise occupied with the hand holding it (i.e. while, climbing, crawling, swimming), and sometimes holds it like a shield, though this functionality is as yet unproven. The mask cannot be thrown, and can only be dropped. A lizard's tongue is capable of ripping the mask from Slugcat's hand, however.

In-Game Lore (spoilers ahead)
The following is dialogue from Big Sister Moon when given a blue pearl from Industrial Complex:

"It's a production record of a mask factory, for what seems to be its last time in service. Have you seen a bone mask? Likely not, they are all gone with their owners. "In ancient times the masks were actually about showing spiritual persuasion - covering the face as a way symbolically abate the self. Then of course, that was quite subverted as excessively ornate and lavish masks became an expression of identity. Some public persons did have problems with narrow doorways.

"Originally monks in a temple would make the masks using bone plaster, and when the production was automated it would generally remain on the same site. So that the old stones could...radiate the material with holiness, I suppose.

"This is from one such facility called Side House, which was here on Pebble's grounds. In the iterator projects many old industrial-religious sites like this were remodeled and incorporated. I think this one was made to provide pellets of holy ash to Pebbles, but knowing him he probably hasn't used much of it!"

Conclusion.

This shows that the owners may have been the vultures and the fact that the masks possess their identity may indicate why they get attacked by fellow vultures if they don't wear their masks. A form of discrimination. But this also says why the lizards fear the vultures, it's in their masks. When a vulture loses his identity, he is forgotten, laughed at and bullied to hell and back.

Trivia

 * Slugcat will turn to mask to the air when eating.
 * Slugcat occasionally holds it like a shield.
 * Masks needs to be held and cannot be swallowed/regurgitated for storage.
 * Scavengers and lizards are less likely to attack you if you have it. Scavengers will try to steal it if given the chance.