The uropygial or preen glands of birds secrete waxes that consist largely of wax
esters. The fatty alcohol components of these are usually relatively simple in nature, consisting
largely of normal C16 and C18 saturated compounds, although those with branched-chains can
make up an appreciable proportion in some species. However, the fatty acids can be highly
complex and are often shorter chain than usual with up to four methyl branches. The main purpose
of the waxes is presumed to be to give a water-proof layer to the feathers, but other functions have
been suggested.