Differential effects of vitamins E and C and carotenoids on growth, resistance to oxidative stress, fledging success and plumage colouration in wild great tits.

Oxidative stress is the imbalance between the production of reactive species and antioxidants, which causes damage to lipids, proteins and DNA. Antioxidants, like vitamins and carotenoids, can limit oxidative damage and can therefore regulate the trade-off between growth, which is a period of high reactive species production, and self-maintenance. However, the role of carotenoids as antioxidants in vivo has been debated, and it has been suggested that carotenoid-based signals indicate the availability of non-pigmentary antioxidants (e.g. vitamins) that protect carotenoids from oxidation, known as the 'protection hypothesis'. To evaluate the importance of vitamins versus carotenoids as antioxidants during growth and to test the protection hypothesis, we supplemented nestling great tits, Parus major, 3, 5 and 7 days after hatching with a single dose of carotenoids and/or vitamins in a 2×2 full-factorial design. We subsequently measured body condition, antioxidant capacity, oxidative damage, fledging success and plumage reflectance. Vitamins enhanced antioxidant capacity, but did not affect oxidative damage. Vitamin-treated nestlings had higher growth rates and higher probability of fledging. In contrast, carotenoids did not affect any of these traits. Furthermore, carotenoid-based colouration increased over the breeding season in nestlings that received vitamins only. This study shows that vitamins are limiting for growth rate and fledging success, and suggests that vitamins could regulate the trade-off between growth and self-maintenance in favour of the former. Moreover, our results are consistent with the idea that carotenoids are minor antioxidants in birds, but they do not support the protection hypothesis.