to focus on the head and neck. It may also help to remove ectoparasites from those hard-to-reach areas. Allopreening is most common among species that are regularly in close physical contact due to flocking or social behaviours, where such contact allows for easier transfer of ectoparasites between individuals. In one study, Macaroni penguins that frequently allopreened had significantly fewer ticks on their heads and necks than those that did not. Green wood hoopoes, a flocking species with a complex hierarchy, show similar frequencies of initiating and reciprocating allopreening of the head and neck regardless of social status, time of year or group size, which suggests that such activity is primarily related to feather hygiene. Most allopreening is done between the two members of a mated pair, and the activity appears to play an important role in strengthening and maintaining pair bonds. It is more common in species where both parents help to raise the offspring and correlates with an increased likelihood that partners will remain together for successive breeding seasons. Allopreening often features as part of the "greeting ceremony" between the members of a pair in species such as albatrosses and penguins, where partners may be separated for a relatively long period of time, and is far more common among sexually monomorphic species (that is, species where the sexes look outwardly similar). It appears to inhibit or redirect aggression, as it is typically the dominant bird that initiates the behaviour. Allopreening appears to reduce the incidence of conflict between members of some colonially living or colonially nesting species. Neighbouring common guillemots that engaged in allopreening were much less likely to fight. Since fights often lead to eggs or chicks being knocked off breeding cliffs, fewer fights led to greater breeding success for allopreening neighbours. Among social flocks of green wood hoopoes, rates of body allopreening (that is, allopreening of another bird's body rather than head and neck) increase with group size. Evidence suggests this type of allopreening reduces social tension, and thus plays an important role in group cohesion. More dominant birds receive far more body allopreening services than do lower-ranked birds, and lower-ranked birds initiate far more body allopreening bouts than do their higher-ranked flock mates. Body allopreening is only reciprocal when done between members of a mated pair; otherwise, the dominant bird reciprocates in fewer than 10% of the instances. Potential problems If birds are exposed to some