Stressful conditions experienced by individuals during their early development have long-term consequences on various life-history traits such as survival until first reproduction. Oxidative stress has been shown to affect various fitness-related traits and to influence key evolutionary trade-offs but whether an individual's ability to resist oxidative stress in early life affects its survival has rarely been tested. In the present study, we used four years of data obtained from a free-living great tit population (Parus major; n = 1658 offspring) to test whether pre-fledging resistance to oxidative stress, measured as erythrocyte resistance to oxidative stress and oxidative damage to lipids, predicted fledging success and local recruitment. Fledging success and local recruitment, both major correlates of survival, were primarily influenced by offspring body mass prior to fledging. We found that pre-fledging erythrocyte resistance to oxidative stress predicted fledging success, suggesting that individual resistance to oxidative stress is related to short-term survival. However, local recruitment was not influenced by pre-fledging erythrocyte resistance to oxidative stress or oxidative damage. Our results suggest that an individual ability to resist oxidative stress at the offspring stage predicts short-term survival but does not influence survival later in life.
Survival until reproduction, as a determinant of individual fitness, strongly depends on the conditions experienced by individuals in their early life . In birds, post-fledging survival is well known to mostly depend on fledging date and body mass at fledging [2,3] but has also recently been shown to depend on cell-mediated immunity  or glucocorticoid-mediated stress response , thereby suggesting a strong influence of physiological processes at early age on future survival.
Oxidative stress, defined as an imbalance between the formation of reactive oxygen species and the antioxidant response in favour of the former , has been identified as a physiological constraint affecting fitness-related traits . The ability of an individual to cope with oxidative stress during early development might have long-lasting consequences for survival, but this has rarely been tested.
At early stages of growth, individuals are hypothesized to be exposed to oxidative stress owing to their fast development and their exposure to intense sibling competition [8,9]. This was empirically demonstrated by manipulations of sibling competition or growth rate, which translated into increased oxidative damage , reduced antioxidant levels  or reduced erythrocyte resistance to oxidative stress , although some studies failed to detect such effects [13,14]. For these reasons, and because oxidative stress has been reported to predict life expectancy in some studies of birds [15,16], one might expect individual resistance to oxidative stress at an early age to explain variance in offspring fledging success and recruitment probability, both major correlates of offspring survival . A link between pre-fledging oxidative damage and recruitment probability was reported in the long-lived European shag Phalacrocorax aristotelis and provides the first piece of evidence of a long-lasting effect of oxidative processes . However, additional tests of this hypothesis on different biological systems are mandatory to draw general conclusions about long-lasting effects of oxidative stress on survival.
In the present study, we used data collected in a free-living great tit population (Parus major; n = 1658 nestlings) over a four-year period to test whether pre-fledging resistance to oxidative stress (measured as erythrocyte resistance to oxidative stress and oxidative damage to lipids) predicts fledging success and/or recruitment probability.
2. Material and methods
Data on offspring resistance to oxidative stress and morphology were collected during spring 2008, 2009 and 2010 in natural populations of great tits breeding in nest-boxes in a forest near Bern, Switzerland (46°7′ N, 7°8′ E). On day 13 post-hatch, we measured offspring body mass (±0.1 g) and took a 30 μl blood sample from the brachial vein to assess their resistance to oxidative stress measured as erythrocyte resistance to oxidative stress (all years, n = 1658) and oxidative damage to lipids (2010 only, n = 790). Nestlings were also sexed using primers 2917/3088 . Fledging occurs on day 17–20 post-hatch, and post-fledging care lasts for about 15–20 days .
Each subsequent breeding season (springs 2009, 2010 and 2011), we captured breeding adults while they were feeding their nestlings using clap-traps in the same forest to assess recruitment probability. Overall, we sampled 1658 nestlings, of which 99 (6.4%) recruited the following year into the same study populations. For data, see the electronic supplementary material.
(a) Erythrocyte resistance to oxidative stress
For all individuals, we assessed erythrocyte resistance to a free-radical attack using the KRL (Kit Radicaux Libres) test (see the electronic supplementary material for details).
(b) Oxidative damage to lipids
For the 790 individuals born in 2010, we also estimated the plasma levels of malondialdehyde (see the electronic supplementary material for details).
(c) Statistical analyses
We tested whether oxidative stress predicted (i) fledging success (whether a nestling fledged or died before fledging) and (ii) recruitment probability (whether a nestling recruited or not the subsequent year) using generalized linear mixed models with a binomial error distribution and a logit link function. Sex of the nestlings, erythrocyte resistance to oxidative stress, oxidative damage, body mass, brood size, laying date, breeding year and the interaction between year and erythrocyte resistance to oxidative stress were included as fixed factors. Identity of the nest was fitted as a random factor and was year-specific to avoid pseudo-replication. For each dependent variable, we ran one model including oxidative damage (log10 transformed), considering year 2010 only, and one model excluding oxidative damage, considering all years.
Fledging success and recruitment probability were strongly positively related to nestling body mass prior to fledging (table 1). Erythrocyte resistance to oxidative stress significantly predicted fledging success with birds with superior pre-fledging erythrocyte resistance to oxidative stress being more likely to fledge (figure 1), but did not predict recruitment probability (table 1). Oxidative damage to lipids did not predict fledging or recruitment probabilities (table 1). Erythrocyte resistance to oxidative stress and oxidative damage to lipids were not significantly correlated (F1,778 = 2.39, p = 0.12), and nor was nestling body mass with any of the measures of oxidative stress (F1,788–1640 < 1.08, p > 0.29).
In a four year study on free-living great tits, we showed that resistance to oxidative stress, measured in terms of erythrocyte resistance to free radical attack, significantly predicted individual fledging success, providing evidence that individual ability to resist oxidative stress predicts short-term survival. However, we did not detect a link between pre-fledging resistance to oxidative stress and local recruitment.
Offspring body mass was a strong predictor of fledging success and recruitment rate in our study, as shown before [2,3], which potentially reflects a competitive advantage before  and/or after  fledging, a higher capacity to resist food shortage  or social dominance .
We found a positive link between pre-fledging erythrocyte resistance to oxidative stress and fledging success, corroborating the few studies reporting a link between resistance to oxidative stress and survival [15,16]. This result has important evolutionary implications because fledging success is a main component of offspring (and parent) fitness , and because it reveals substantial selection pressure on juvenile resistance to oxidative stress. Furthermore, given that most nestling mortality occurs before day 13 post-hatch, a stronger relationship may be expected between fledging success and resistance to oxidative stress measured shortly after hatching. Nestling resistance to oxidative stress likely depends on their ability to monopolize antioxidant resources provided by the parents , but also on individual growth rate , or on the presence of parasites , or environmental pollution .
Pre-fledging resistance to oxidative stress did not predict recruitment probability, suggesting that greater nestling resistance to oxidative stress does not provide post-fledging selective advantage in our study system. This result contrasts with a recent study on the long-lived shag . The discrepancy might reflect the strong post-fledging selection on body condition in our model species and/or the extremely high predation risk occurring shortly after fledging in passerines , which may mask any advantage of greater nestling resistance to oxidative stress. Alternatively, our results could reflect an absence of correlation between pre- and post-fledging resistance to oxidative stress, which deserves further investigation.
Our study cannot discriminate between dispersal and post-fledging survival. However, given the absence of evidence for dispersal beyond our studied local population despite intensive sampling in several surrounding populations over 20 years (H. Richner 1992–2012, unpublished data), the well-known preference of great tits for nest-boxes over natural holes , and the fact that some of the nest-boxes remain empty every year, it is unlikely that we missed many recruits. Therefore, local recruitment may be taken as a reasonable proxy for survival to breeding age.
In summary, our study provides evidence in a wild bird for selection acting before but not after fledging on the basis of nestling ability to resist oxidative stress and suggests that some components of resistance to oxidative stress are related to short-term survival during offspring growth.
The study was financially supported by the Swiss National Science Foundation. S.L. is supported by a Swiss NSF post-doctoral fellowship (grant no. PBBEP3_139396), and J.D.B. by a Royal Society University Research Fellowship.
- Received September 21, 2012.
- Accepted October 2, 2012.
- © 2012 The Author(s) Published by the Royal Society. All rights reserved.