learned component. Birds that are hand-reared without access to a role model have abnormalities in their preening behaviours. Despite spending considerable time in their efforts, they do not use proper techniques to groom effectively and may do a poor job overall as a result. Displaced feathers can cause birds considerable trouble; such feathers might become damaged, could interrupt the smooth flow of air over a flying bird, or might allow the bird's body heat to escape. Preening allows a bird to reposition such displaced feathers. There is evidence that filoplumes, specialised feathers buried under a bird's outer covering of contour feathers, help to signal when contour feathers have been displaced. Mechanoreceptors at the base of the filoplumes only fire when contour feathers are displaced or the filoplume moves. Preening enables birds to remove dirt and parasites from their plumage, and assists in the waterproofing of feathers. During moult, birds remove the sheaths from around their emerging pin feathers while preening. Because feathers are critical to a bird's survival – contributing to insulation, waterproofing and aerodynamic flight – birds spend a great deal of time maintaining them. When resting, birds may preen at least once an hour. Studies on multiple species have shown that they spend an average of more than 9% of each day on maintenance behaviours, preening occupying over 92% of that time, though this figure can be significantly higher. Studies found that some gull species spent 15% of daylight hours during the breeding season preening, while another showed that common loons spent upwards of 25% of their day preening. In most of the studied species where the bird's sex could be determined in the field, males spent more time preening than females, though this was reversed in ducks. Some ratites, which are not dependent on their feathers for flight, spend far less time on maintenance behaviours. One study of ostriches found that they spent less than 1% of their time engaged in such behaviours. Preen oil Fully grown feathers are essentially dead structures, so it is vital that birds have some way to protect and lubricate them. Otherwise, age and exposure cause them to become brittle. To facilitate that care, many bird species have a preen or uropygial gland, which opens above the base of the tail feathers and secretes a substance containing fatty acids, water, and waxes. The bird gathers this substance on its bill and applies