Data from: Baseline glucocorticoids are drivers of body mass gain in a diving seabird

Life-history trade-offs are influenced by variation in individual state,
with individuals in better condition often completing life-history stages
with greater success. Although resource accrual significantly impacts key
life-history decisions such as the timing of reproduction, little is known
about the underlying mechanisms driving resource accumulation. Baseline
corticosterone (CORT, the primary avian glucocorticoid) mediates daily and
seasonal energetics, responds to changes in food availability, and has
been linked to foraging behavior, making it a strong potential driver of
individual variation in resource accrual and deposition. Working with a
captive colony of white-winged scoters (Melanitta fusca deglandi), we
aimed to causally determine whether variation in baseline CORT drives
individual body mass gains mediated through fattening rate (plasma
triglycerides corrected for body mass). We implanted individuals with each
of three treatment pellets to elevate CORT within a baseline range in a
randomized order: control, low dose of CORT, high dose of CORT, then blood
sampled and recorded body mass over a two-week period to track changes in
baseline CORT, body mass, and fattening rates. The high CORT treatment
significantly elevated levels of plasma hormone for a short period of time
within the biologically relevant, baseline range for this species, but
importantly did not inhibit the function of the HPA
(hypothalamic–pituitary–adrenal) axis. Furthermore, an elevation in
baseline CORT resulted in a consistent increase in body mass throughout
the trial period compared to controls. This is some of the first empirical
evidence demonstrating that elevations of baseline CORT within a
biologically relevant range have a causal, direct, and positive influence
on changes in body mass.