Complex sentence: Many birds have a preen-or uropygial gland opening above the base of the tail feathers and producing a lipid rich secretion that is first applied to the bill and then to the feathers.
Split into simpler sentences: Many birds have a preen-or uropygial gland opening above the base of the tail feathers. This gland produces a lipid rich secretion that is applied first to the bill and then to the feathers.