We hypothesized that increasing chick plasma testosterone concentrations, t
ransmitted from the mothers via their eggs, enhances survival of their offs
pring and that the fitness of the young, depending on the maternal hormones
, is influenced by parental quality. To test our hypotheses we distinguishe
d the broods of white storks Ciconia ciconia L. where chicks died and those
where all chicks survived, We analysed the plasma testosterone concentrati
ons in the chicks, the ability of the chicks to be first to receive food an
d the mass of chicks before fledging in relation to their hatching order an
d recorded the body mass of parents and food mass delivered by them.
Female storks used the asymmetries in testosterone concentrations within a
brood to control brood size and adjusted the number of young hatched to mat
ch the parental ability to rear offspring. Females of poor condition altere
d the testosterone concentrations to produce large differences between the
chicks: The first-hatched chicks, which had high plasma testosterone levels
, responded faster to the feeding parent and received more food than did th
eir younger siblings. One or two later-hatched chicks, which had lower test
osterone levels, died in these broods. Females in good condition produced s
mall differences in testosterone concentrations between the chicks and all
chicks survived in their brood. Chicks that were raised by the females of p
oor condition in reduced broods were heavier than chicks that were raised b
y females of good condition in broods where all chicks survived.
We suggest that the control of brood size by testosterone concentration, tr
ansmitted by the mother to the chicks, is a hormonal means of condition-dep
endent reproductive strategy in the white stork.