Intraspecific female aggression during the breeding season can have se
veral different functions: defence of resources, defence against intra
specific brood parasitism and defence of mating status. The intraspeci
fic aggressive behaviour of breeding female starlings, Sturnus vulgari
s, was examined by exposing them to a simulated intrusion of a conspec
ific bird. A caged male or female starling was placed close to the nes
t of a breeding pair. Aggressiveness was scored as the proportion of t
ime birds spent near the caged birds after discovery. Caged females el
icited stronger responses from females than caged males. Females sang
at caged females and sometimes also attacked them. They were most aggr
essive towards them during the pre-laying period and less so during th
e egg-laying, incubation and nestling periods. Females were more aggre
ssive towards a caged female when their mate had access to an addition
al nestbox to which he could attract an additional female, then when h
e had not. A time-budget study demonstrated that females spent more ti
me near their nest site during the pre-laying period when their mate h
ad access to an additional nestbox than when he had not. These pattern
s are most consistent with females trying to secure male brood-rearing
assistance by preventing or delaying the settlement of secondary fema
les, since early established secondary females may compete for male he
lp in incubating eggs and feeding nestlings. (C) 1997 The Association
for the Study of Animal Behaviour.