| Summary: | 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.
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