Owners of feather-bare ex-bats will sometimes report a strange growth or tumour at the base of the hen’s tail. This is a normal part of a hen’s anatomy, called the uropygial gland.
The uropygial gland, more commonly known as the ‘oil gland’, plays an important role in helping the hen to keep her feathers waterproof and in good condition. In hens with full feathering the gland is less obvious but may suddenly become visible during a heavy moult.
The gland is found in most birds and is best developed in water birds, and the secretions of the gland empty onto the skin surface through one or more nipple-like pores. Most birds preen by rubbing their head and beak over the preen gland pore and then distribute the accumulated oil over the feathers of the body and wings, skin and legs.