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dead structures, so it is vital that birds have some way to protect and lubricate them. Otherwise, age and exposure cause them to become brittle. To facilitate that care, many bird species have a preen or uropygial gland, which opens above the base of the tail feathers and secretes a substance containing fatty acids, water, and waxes. The bird gathers this substance on its bill and applies it to its feathers. The gland is generally larger (in relation to body size) in waterbirds, including terns,

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