  Oxidative stress occurs when reactive oxygen species (ROS) exceed antioxidant
defences, which can have deleterious effects on cell function, health and
survival. Therefore, organisms are expected to finely regulate pro-oxidant and
antioxidant processes. ROS are mainly produced through aerobic metabolism and
vary in response to changes in energetic requirements, whereas antioxidants may
be enhanced, depleted or show no changes in response to changes in ROS levels.
We investigated the repeatability, within-individual variation and correlation
across different conditions of two plasmatic markers of the oxidative balance
in 1108 samples from 635 free-living adult collared flycatchers (Ficedula
albicollis). We sought to manipulate energy constraints by increasing wing load
in 2012 and 2013 and by providing additional food in 2014. We then tested the
relative importance of within- and between-individual variation on reactive
oxygen metabolites (ROMs), a marker of lipid and protein peroxidation, and on
non-enzymatic antioxidant defences (OXY test). We also investigated whether the
experimental treatments modified the correlation between markers. Antioxidant
defences were repeatable (range of repeatability estimates = 0.128--0.581),
whereas ROMs were not (0--0.061). Antioxidants varied neither between
incubation and nestling feeding nor between sexes. ROMs increased from
incubation to nestling feeding in females and were higher in females than
males. Antioxidant defences and ROM concentration were globally positively
correlated, but the correlation varied between experimental conditions and
between years. Hence, the management of oxidative balance in wild animals
appears flexible under variable environmental conditions, an observation which
should be confirmed over a wider range of markers.
