Figure 1. Effects of oxidative load and carotenoid intake on beak colouration, circulating carotenoids, body mass and blood redox state.

Birds were exposed to either low (ROS−) or high oxidative load (ROS+) and to either low (CAR−) or high carotenoid intake (CAR+). Bars represent group means controlled for pre-experimental values and their standard errors obtained from the models in Table 1. Shown are model P-values for the main effects of oxidative challenge (ROS) and carotenoid intake (CAR), as well as their interaction (INT). (a,b) Both, beak red chroma (higher values = more saturated red colour) and total plasma carotenoids were reduced by oxidative challenge and enhanced by high carotenoid intake. (c) Neither treatment factor had a significant effect on body mass, though their interaction resulted in body mass in ROS + CAR + group that was lower than would be expected if both effects were additive. None of the treatment groups, however, differed from the control (Tukey’s post-hoc test on change scores: P ≥ 0.379). (d–f) Oxidative challenge elicited a marked increase in both, lipophilic (ZE/tHODE) and hydrophilic (OXY) antioxidant capacity, resulting in marginally insignificant increase in blood oxidative damage (8-isoprostane). Carotenoids counteracted the effect of oxidative challenge on the activity of other lipophilic antioxidants (shown as reduced ZE/tHODE ratio) while having no effect on oxidative damage levels. On the other hand, hydrophilic antioxidant capacity was unaffected by lipophilic carotenoids.