Data from: Biomarker of burden: feather corticosterone reflects energetic expenditure and allostatic overload in captive waterfowl

1.Allostatic load describes the interplay between energetic demand and
availability and is highly context dependent, varying between seasons and
life history stages. When energy demands exceed physiological set points
modulated by glucocorticoid hormones, individuals may experience
allostatic overload and transition between stages in sub-optimal
physiological states. 2.Corticosterone, the major glucocorticoid hormone
regulating energy expenditure in birds, is incorporated into growing
feathers (CORTf), and it has been suggested that CORTf reflects long-term
records of allostatic load during feather growth. However, relationships
between allostatic load and CORTf have not been adequately evaluated. If
such relationships exist, the use of CORTf to investigate cross-seasonal
effects could provide novel insights into impacts of past allostatic load
and/or overload events. 3.We tested whether experimental increases in
daily workload during two adjacent life history stages would be reflected
in CORTf levels, and examined if CORTf levels reflected either current
energetic demand or allostatic load prior to feather growth. 4.Daily
workloads in female mallard (Anas platyrhynchos) ducklings were increased
over a 6-week period using physical obstacles and/or carrying back-mounted
weights. We measured daily energy expenditure, growth, body mass, and
CORTf in growing ducklings. Then, we induced feather moult and reapplied
combinations of workload treatments for an additional 6 weeks to
investigate whether effects of past energetic demands would be detected in
future CORTf levels. 5.Ducklings confronted with higher workloads during
development had reduced body mass, growth rates, and consequently higher
daily energy expenditure and CORTf values compared to controls. When
ducklings were fully developed, CORTf patterns in birds re-exposed to
workload treatments reflected only current, rather than past, energetic
demands. However, under allostatic overload conditions, past levels of
CORTf were associated with CORTf in the subsequent moult. 6.Our study
confirms the previously untested assumption that CORTf reflects energetic
demand during the period of feather growth in a precocial bird. We show
that allostatic overload conditions early in life, which temporarily
supress growth, can be detected using CORTf; an event potentially missed
in studies which rely solely on measures of body condition alone. 7.We
suggest that CORTf can provide a valuable biomarker of allostatic load and
overload conditions during the period of feather growth, but highlight how
context should be considered for studies using CORTf to investigate
influences of carry-over effects. Our study contributes to building a
physiological foundation to inform interpretations of ecological patterns
using CORTf.