some cases, it is done in place of another activity that birds are strongly motivated, but unable, to do. In one study, black-headed gulls which were prevented from incubating a full clutch of eggs (by the removal of eggs from their nest) responded by preening and nest building – both displacement activities. When all three eggs in their regular clutch were removed, the gulls showed a significant increase in the amount of time they spent preening. The conflict between two incompatible drives, such as incubating and escape, can lead a bird to engage in displacement activities. Nesting Sandwich and common terns preen when they have been alarmed by a potential predator or when they have had an aggressive encounter with a neighbouring bird, for instance. Fighting European starlings will break off their battles to preen. Allopreening Although preening is primarily an individual behaviour, some species indulge in allopreening, one individual preening another. It is not particularly common among birds, though species from at least 43 families are known to engage in the mutual activity. Most allopreening activity concentrates on the head and neck, a lesser amount being directed towards the and and an even smaller percentage applied to the . A few species are known to allopreen other areas, including the rump, tail, belly and underwing. Several hypotheses have been advanced to explain the behaviour: that it assists in effective grooming, that it assists in recognition of individuals (mates or potential sexual partners), and that it assists in social communication, reducing or redirecting potential aggressive tendencies. These functions are not mutually exclusive. Evidence suggests that different species may participate for different reasons, and that those reasons may change depending on the season and the individuals involved. In most cases, allopreening involves members of the same species, although some cases of interspecific allopreening are known; the vast majority of these involve icterids, though at least one instance of mutual grooming between a wild black vulture and a wild crested caracara has been documented. Birds seeking allopreening adopt specific, ritualised postures to signal so; they may fluff their feathers out or put their heads down. Captive birds of social species that normally live in flocks, such as parrots, will regularly solicit preening from their human owners. There is some evidence that allopreening may help to keep in good condition those feathers that a bird cannot easily reach by itself; allopreening activities tend