Data from: Endocrine phenotype, reproductive success and survival in the great tit, Parus major

A central goal in evolutionary ecology is to characterize and identify
selection patterns on the optimal phenotype in different environments.
Physiological traits, such as hormonal responses, provide important
mechanisms by which individuals can adapt to fluctuating environmental
conditions. It is therefore expected that selection shapes hormonal
traits, but the strength and the direction of selection on plastic
hormonal signals are still under investigation. Here, we determined
whether, and in which way, selection is acting on the hormones
corticosterone and prolactin by characterizing endocrine phenotypes and
their relationship with fitness in free-living great tits, Parus major. We
quantified variation in circulating concentrations of baseline and
stress-induced corticosterone and in prolactin during the prebreeding
(March) and the breeding season (May) for two consecutive years, and
correlated these with reproductive success (yearly fledgling number) and
overwinter survival in female and male individuals. In both years,
individuals with high baseline corticosterone concentrations in March had
the highest yearly fledgling numbers; while in May, individuals with low
baseline corticosterone had the highest yearly reproductive success.
Likewise, individuals that displayed strong seasonal plasticity in
baseline corticosterone concentrations (high in March and low in May) had
the highest reproductive success in each year. Prolactin concentrations
were not related to reproductive success, but were positively correlated
to the proximity to lay. Between-year plasticity in stress-induced
corticosterone concentrations of males was related to yearly variation in
food abundance, but not to overall reproductive success. These findings
suggest that seasonally alternating directional selection is operating on
baseline corticosterone concentrations in both sexes. The observed
between-year consistency in selection patterns indicates that a one-time
hormone sample in a given season can allow the prediction of individual
fitness.