In many polygynous bird species, males allocate most of their parental
effort to their primary females' broods. There are several hypotheses
that may explain this: the relative reproductive value of the brood,
the energetic demand of the brood, the genetic quality of the female a
nd the certainty of fatherhood may all be higher for the primary femal
es' broods. Since these parameters may covary in nature, experiments a
re necessary to determine their importance. Bigynous male European sta
rlings, Sturnus vulgaris, predominantly incubated the eggs and fed the
nestlings of their primary females. In an experiment we altered the o
rder in which the clutches hatched by exchanging the primary and secon
dary females' eggs before hatching. Even though experimental males mos
tly incubated their primary females' clutches, they predominantly fed
the nestlings of the secondary female. Experimental males fed secondar
y females' nestlings significantly more than control males did. In fac
t, experimental males invested in their secondary females' broods to t
he same extent as control males invested in their primary females' bro
ods. This result demonstrates that males use relative brood age to dec
ide how to allocate their parental effort between their broods which h
as important implications for the evolution of the starling mating sys
tem. (C) 1997 The association for the Study of Animal Behaviour.