A relatively large, compact, bilobed, secretory organ located at the base of the tail (uropygium) of most birds having a keeled sternum. It is known also as the preen, oil, or scent gland. This is the only true skin gland possessed by this class of vertebrates. The gland develops from a pair of invaginations of the dorsal ectoderm on each side of the free coccygeal vertebrae, giving rise to numerous secretory tubules surrounded by a sheath of connective tissue of mesodermal origin. Experiments with duck embryos have shown that formation of the gland is dependent upon a specific action of the subjacent mesoderm on the overlying ectoderm. Implanted mesoderm from other body regions failed to induce a gland-forming response in the ectoderm.