in females may be involved in the production and secretion of lipids with female pheromone activity. Waterproofing effect 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. 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 are not supported by scientific studies. Antiparasitic effect The taxonomic richness of avian louse burdens covaries positively with uropygial gland size (relative to body size) across avian taxa, suggesting coevolution between gland size and parasite biodiversity. The hoopoe (Upupa epops) uropygial gland harbours symbiotic bacteria whose excretions reduce the activity of feather-degrading bacteria and thus help to preserve the plumage. In vitro studies suggest that the preen oils of rock doves (Columba livia) are effective against lice, however in vivo studies which removed the preen gland from captive birds had no significant effect on louse loads over the course of four months (but did reduce plumage quality), suggesting that preen oil may not be an important defense against lice in this species. Similarly, there was no evidence to support a role of the gland secretion in attracting biting midges and black flies in blue tits (Cyanistes caeruleus) and feral pigeons (Columba livia). Cosmetic effect Secretions of the uropygial gland of greater flamingos (Phoenicopterus roseus) contain carotenoids, organic pigments which give flamingos their pink colour. During the breeding season, greater flamingos increase the frequency of their spreading uropygial secretions over their feathers and thereby enhance their colour. This cosmetic use of uropygial secretions has been described as applying "make-up". The chemical composition The biochemical composition of preen oil has been reported to significantly vary among birds in several ways: for example, the qualitative and quantitative content of the volatile compounds, chain length of diols, lipid concentration, ester saturation, type of fatty acids and their percentage composition. This variation is due to a range of factors, including species, age, sex and season. In chickens, a limited number of studies have investigated the specific effect of diet on the fatty acid composition of preen oil in meat chickens. The preen oil of meat chickens