PippinTheChicken
🧡🤍💙
That's the preen gland. It contains an oily substance that birds use to preen their feathers and clean themselves. If you even see them rubbing their beak (or even biting) the base of their tail they're covvering their beak with the oil so they can preen
Here's a video:
(Not my video)
Here's a video: