The uropygial gland of birds protects them from water penetration, fungi, and bacteria by producing preen waxes.
"In addition to the stratum corneum barrier, glandular lipids are deposited exteriorly to the epidermis in both mammals and birds (Hadley, 1991)…In birds, 'preen waxes' from the uropygial gland are spread over feathers to prevent water penetration and ingress of bacteria and fungi. Uropygial secretions contain a complex mixture of lipids in which wax esters usually predominate…In birds and mammals, plumage and pelage appear to impede significantly the passage of water vapor from skin to atmosphere, although the skin remains the principal barrier to TEWL [transepidermal water loss] (Cena and Clark, 1979; Webster et al., 1985). In pigeons, for example, plumage contributes 5–20% of total resistance to water loss through the integument, and the plumage and boundary layer together account for 6–26% of total resistance to water vapor diffusion (Webster et al., 1985). Therefore, adjustments of plumage or pelage and seasonal shedding patterns are potential means of adjusting rates of TEWL." (Lillywhite 2006:219)
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