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In page Uropygial gland:

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The uropygial gland is strongly developed in many waterbirds, such as ducks, petrels, pelicans and in the osprey and the oilbird. A study examining the gland's mass relative to body weight in 126 bird species showed the absence of a clear-cut correlation between the gland's mass and the degree of birds' contact with water.[1][2] Anecdotal reports indicating that the waterproofing effect of the hydrophobic uropygiols might be increased by electrostatic charge to the oiled feather through the mechanical action of preening [3] are not supported by scientific studies.[4]