Tab at the base of a chickens tail

[millietime]

Ok I am not sure where to put this chicken anatomy question so I am putting it here. What is the tab at the base of a chickens tail? It almost looks like a big skin tab or pin feather coming up. All of my girls have this and I cannot for the life of me find what it is or what it's for?! Any insight would be greatly appreciated!

 

[laircy]

It's an oil gland that they rub when they're preening their feathers. Some all-natural conditioner, I suppose.

 

[millietime]

Laircy- thank you so much for clarifying this for me!!!