Data from: Do glucocorticoids predict fitness? Linking environmental conditions, corticosterone and reproductive success in the blue tit, Cyanistes caeruleus

Glucocorticoids, including corticosterone (CORT), have been suggested to
provide a physiological link between ecological conditions and fitness.
Specifically, CORT, which is elevated in response to harsh conditions, is
predicted to be correlated with reduced fitness. Yet, empirical studies
show that CORT can be non-significantly, positively and negatively linked
with fitness. Divergent environmental conditions between years or study
systems may influence whether CORT is linked to fitness. To test this, we
monitored free-living blue tits (Cyanistes caeruleus) during breeding over
3 years. We quantified foraging conditions during brood rearing, and
examined whether they were correlated with parental baseline CORT and
reproductive success. We then tested whether CORT predicted fitness.
Elevated parental CORT was associated with lower temperatures, greater
rainfall and lower territory-scale oak density. Whereas asynchrony with
the caterpillar food peak was correlated with reduced nestling mass and
fledging success, but not parental CORT. Only low temperatures were
associated with both reduced nestling mass and elevated parental CORT.
Despite this, parents with elevated CORT had lighter offspring in all
years. Contrarily, in 2009 parental CORT was positively correlated with
the number fledged. The absence of a direct link between the foraging
conditions that reduce nestling quality and elevate parental CORT suggests
that parental CORT may provide a holistic measure of conditions where
parents are working harder to meet the demands of developing young. As the
positive correlation between parental CORT and fledging success differed
between years, this suggests that contrasting conditions between years can
influence correlations between parental CORT and fitness. Ultimately, as
CORT concentrations are intrinsically variable and linked to the prevalent
conditions, studies that incorporate environmental harshness will improve
our understanding of evolutionary endocrinology.