Aggression and microhabitat segregation among nesting common terns Sterna hirundo and Mediterranean gulls Larus melanocephalus

We studied aggressive behaviour in a mixed colony of Mediterranean gulls and common terns. The two species bred
syntopically but selected different reproductive microhabitats. Common terns were more aggressive than Mediterranean gulls, and
aggression rate was higher towards heterospecifics than conspecifics. In both species, aggression rates were higher during the incubation
phase, fell to a minimum when chicks rested in nests and increased again when chicks were able to wander inside the colony.
High aggression rates during the pre-hatching phase are in agreement with theories of parental investment, while those involving
adults during the chick wandering phase could be explained by a higher encounter rate with non-familiar adults, that induced an
increase of aggressions between territory holders. The lower aggression in the “early chicks” phase can be explained by the
increased food demand by chicks and the consequent reduction of adult density foraging out of the colony; however, selection of
differing reproductive microhabitat can be considered as a further factor promoting coexistence, enhancing chicks survival immediately
after hatching.