the method used by a species may be related to its ecology. For instance, New World warblers that are primarily arboreal tend to be overwing scratchers, while those that spend significant time on the ground are typically underwing scratchers. In general, preening takes place while the bird is perched, on the ground, or swimming, but some of the more aerial species (including swifts, swallows, terns and albatrosses) preen while flying. Many birds have a slight overhang at the tip of their upper mandible. Experiments suggest that this allows birds to apply shearing forces that kill the flat-bodied feather lice; the removal of the bill tip caused an increase in feather lice due to ineffective preening. Preening is often done in association with other maintenance behaviours, including bathing, dusting, sunning, oiling or anting, and can either precede or follow these other behaviours. All birds typically preen after bathing. Groups of birds sometimes all groom individually at the same time. This has been seen in species ranging from herons to blackbirds. Secondary functions Preening may help to send sexual signals to potential mates because plumage colouration (which can be altered by the act of preening) can reliably reflect the health or "quality" of its bearer. In some species, preen oil is used to cosmetically colour the plumage. During the breeding season, the preen oil of the great white pelican becomes red-orange, imparting a pink flush to the bird's plumage. The preen oil of several gull and tern species, including Ross's gull, contains a pink colourant which does the same. The heads of these birds typically show little pink, because of the difficulty of reaching those areas with preen oil. The yellow feathers of the great hornbill are also cosmetically coloured during preening. The preen oil of the Bohemian waxwing increases the UV reflectance of its feathers. Ritualised preening is used in courtship displays by several species, particularly ducks; such preening is typically designed to draw attention to a modified structure (such as the sail-shaped secondaries of the drake mandarin duck) or distinctive colour (such as the ) on the bird. Mallards of both sexes will lift a wing so that the brightly coloured speculum is showing, then will place their bill behind the speculum as if preening it. Courtship preening is more conspicuous than is preening for feather maintenance, using more stereotypical movements. Preening may be performed as a displacement activity. In