Female aggression in the European Starling during the breeding season
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Intraspecific female aggression during the breeding season can have several different functions: defence of resources, defence against intraspecific brood parasitism and defence of mating status. The intraspecific aggressive behaviour of breeding female starlings, Sturnus vulgaris, was examined by exposing them to a simulated intrusion of a conspecific bird. A caged male or female starling was placed close to the nest of a breeding pair. Aggressiveness was scored as the proportion of time birds spent near the caged birds after discovery. Caged females elicited stronger responses from females than caged males. Females sang at caged females and sometimes also attacked them. They were most aggressive towards them during the pre-laying period and less so during the egg-laying, incubation and nestling periods. Females were more aggressive towards a caged female when their mate had access to an additional nestbox to which he could attract an additional female, then when he had not. A time-budget study demonstrated that females spent more time near their nest site during the pre-laying period when their mate had access to an additional nestbox than when he had not. These patterns are most consistent with females trying to secure male brood-rearing assistance by preventing or delaying the settlement of secondary females, since early established secondary females may compete for male help in incubating eggs and feeding nestlings.