it to its feathers. The gland is generally larger (in relation to body size) in waterbirds, including terns, grebes and petrels. However, studies have found no clear correlation between the size of the gland and the amount of time a species spends in the water; it is not consistently largest in those species that spend the most time in the water. Preen oil plays a role in reducing the presence of parasitic organisms, such as feather-degrading bacteria, lice and fungi, on a bird's feathers. One study of Eurasian hoopoes showed that the presence of symbiotic bacteria (Enterococcus faecalis) in their preen oil inhibited the growth of the feather-degrading bacteria Bacillus licheniformis. Enterococcus faecalis did this by releasing a bacteriocin. Female hoopoes transfer preen oil onto their brood patches and eggs, which results in the transfer of bacteria as well. Preen oil and bacteria are rubbed into microscopic pits on the surface of the eggs during incubation. This alters the colour of the eggs (darkening them) but there is also evidence that the bacteria may help to protect the developing chicks. Other studies have shown that removing or restricting access to the uropygial gland typically results in a higher bacterial parasite load on the plumage, though not necessarily of feather-degrading bacterial species. Preen oil may play a part in protecting at least some species from some internal parasites; a study of the incidence of avian malaria in house sparrows found that uninfected birds had larger uropygial glands and higher antimicrobial activity in those glands than infected birds did. There is even evidence that the foul-smelling preen oil of hoopoes and wood hoopoes may help to repel mammalian predators. Preen oil helps to maintain the waterproofing of a bird's plumage. Though the oil does not provide any direct waterproofing agent, it helps to extend the life of the feather – including the microscopic structures (the barbs and barbules) which interlock to create the waterproof barrier. While most species have a preen gland, the organ is missing in the ratites (emu, ostriches, cassowaries, rheas and kiwis) and some neognath birds, including bustards, woodpeckers, a few parrots and pigeons. Some species that lack a preen gland instead have powder down feathers which continually break down into a fine dust that the birds apply to their contour feathers while preening. These powder down feathers may be scattered throughout the bird's plumage or concentrated into dense