What is this weird little lump near the tail of all my quail?

[Majak5]

There is this weird little lump thing just above the tail of each of my quail and I have no idea what it is but I do see my quail try scratch st it sometimes. Anyone know what it is?

 

[The Moonshiner]

Google
"Uropygial gland"

 

[Rubysword]

It's a gland which secretes oil, which the quail use to preen their feathers with, which is why they peck at it.