Resources allocated in reproduction are traded off against those invested in self-maintenance such as antioxidant response. Glutathione (GSH) is an intracellular antioxidant defence that scavenges reactive oxygen species, the deleterious byproducts of oxygen consumption. Given the role of intracellular GSH in pheomelanogenesis, a trade-off in GSH allocation between resistance to oxidative stress and melanin production may take place. To investigate how intracellular GSH is regulated in differently coloured individuals at the time of reproduction (in adults) and of intense melanogenesis (in nestlings), we measured the total pool of GSH produced ( t GSH), consumed (oxGSH) and available (redGSH) in adult tawny owls and their offspring which were cross-fostered between randomly chosen nests. Our goal was to describe potential correlations between resistance in oxidative stress and colour morphs in natural conditions. Nestling GSH levels were correlated with GSH levels in their genetic and foster parents suggesting that producing GSH is genetically and environmentally determined (although the effect of the foster nest seemed stronger). This species shows continuous variation in phe - omelanin reddish colouration, which is associated with life-history strategies. Based on the hypothesis of GSH dependence of pheomelanism, we expected a greater amount of oxGSH in dark compared to light pheomelanic nestlings, which was not the case. In contrast, light melanic breeding adults had higher levels of t GSH and redGSH than dark breeders probably because of a higher investment in antioxidant capacity. The link between pheomelanism and GSH may therefore be due to the fact that differently colored individuals have different life-history strategies rather than because the production of pheomelanin pigments requires GSH.

A growing body of studies is showing that offspring telomere length (TL) can be influenced by the age of their parents. Such a relationship might be explained by variation in TL at conception (gamete effect) and/or by alteration of early growth conditions in species providing parental care. In a long-lived bird with bi-parental care, the Alpine swift (Apus melba), we exchanged an uneven number of 2 to 4-day-old nestlings between pairs as part of a brood size manipulation. Nestling TL was measured at 50 days after hatching, which allowed investigation of the influence of the age of both their biological and foster parents on offspring TL, after controlling for the manipulation. Nestling TL was negatively related to the age of their biological father and foster mother. Nestling TL did not differ between enlarged and reduced broods. These findings suggest that offspring from older males were fertilized by gametes with shorter telomeres, presumably due to a greater cell division history or a longer accumulation of damage, and that older females may have provided poorer parental care to their offspring.

Dispersal entails costs and might have to be traded off against other life-history traits. Dispersing and philopatric individuals may thus exhibit alternative life-history strategies. Importantly, these differences could also partly be modulated by environmental variation. Our previous results in a patchy population of a small passerine, the collared flycatcher, suggest that, as breeding density, a proxy of habitat quality, decreases, dispersing individuals invest less in reproduction but maintain a stable oxidative balance, whereas philopatric individuals maintain a high reproductive investment at the expense of increased oxidative stress. In this study, we aimed at experimentally testing whether these observed differences between dispersing and philopatric individuals across a habitat quality gradient were due to food availability, a major component of habitat quality in this system. We provided additional food for the parents to use during the nestling rearing period and we measured subsequent parental reproductive effort (through provisioning rate, adult body mass, and plasmatic markers of oxidative balance) and reproductive output. Density-dependent differences between dispersing and philopatric parents in body mass and fledging success were observed in control nests but not in supplemented nests. However, density-dependent differences in oxidative state were not altered by the supplementation. Altogether, our results support our hypothesis that food availability is responsible for some of the density-dependent differences observed in our population between dispersing and philopatric individuals but other mechanisms are also at play. Our study further emphasizes the need to account for environmental variation when studying the association between dispersal and other traits.

Although disruption of glucose homeostasis is a hallmark of ageing in humans and laboratory model organisms, we have little information on the importance of this process in free-living animals. Poor control of blood glucose levels leads to irreversible protein glycation. Hence, levels of protein glycation are hypothesized to increase with age and to be associated with a decline in survival. We tested these predictions by measuring blood glycated haemoglobin in 274 adult collared flycatchers of known age and estimating individual probability of recapture in the following 2 years. Results show a strong decrease in glycated haemoglobin from age 1 to 5 years and an increase thereafter. Individuals with high levels of glycated haemoglobin had a lower probability of recapture, even after controlling for effects of age and dispersal. Altogether, our findings suggest that poor control of glucose homoeostasis is associated with lower survival in this free-living bird population, and that the selective disappearance of individuals with the highest glycation levels could account for the counterintuitive age-related decline in glycated haemoglobin in the early age categories.

Dispersing and non-dispersing individuals often differ in phenotypic traits (e.g. physiology, behaviour), but to what extent these differences are fixed or driven by external conditions remains elusive. We experimentally tested whether differences in nest-defence behaviour between dispersing and non-dispersing individuals changed with local habitat quality in collared flycatchers, by providing additional food during the nestling rearing period. In control (non-food-supplemented) nests, dispersers were less prone to defend their brood compared with non-dispersers, whereas in food-supplemented nests, dispersing and non-dispersing individuals showed equally strong nest defence. We discuss the importance of dispersal costs versus adaptive flexibility in reproductive investment in shaping these differences in nest-defence behaviour between dispersing and non-dispersing individuals. Irrespective of the underlying mechanisms, our study emphasizes the importance of accounting for environmental effects when comparing traits between dispersing and non-dispersing individuals, and in turn assessing the costs and benefits of dispersal.

Fitness can be profoundly influenced by the age at first reproduction (AFR), but to date the AFR-fitness relationship only has been investigated intraspecifically. Here we investigated the relationship between AFR and average lifetime reproductive success (LRS) across 34 bird species. We assessed differences in the deviation of the Optimal AFR (i.e., the species-specific AFR associated with the highest LRS) from the age at sexual maturity, considering potential effects of life-history as well as social and ecological factors. Most individuals adopted the species-specific Optimal AFR and both the mean and Optimal AFR of species correlated positively with lifespan. Interspecific deviations of the Optimal AFR were associated with indices reflecting a change in LRS or survival as a function of AFR: a delayed AFR was beneficial in species where early AFR was associated with a decrease in subsequent survival or reproductive output. Overall, our results suggest that a delayed onset of reproduction beyond maturity is an optimal strategy explained by a long lifespan and costs of early reproduction. By providing the first empirical confirmations of key predictions of life-history theory across species, this study contributes to a better understanding of life-history evolution.

Iteroparous organisms maximise their overall fitness by optimising their reproductive effort over multiple reproductive events. Hence, changes in reproductive effort are expected to have both short- and long-term consequences on parents and their offspring. In laboratory rodents, manipulation of reproductive efforts during lactation have however revealed little short-term reproductive adjustments, suggesting that female laboratory rodents might express maximal rather than optimal levels of reproductive investments as observed in semelparous organisms. Using a litter size manipulation (LSM) experiment in a small wild-derived rodent (the common vole; Microtus arvalis), we show that females altered their reproductive efforts in response to LSM, with females having higher metabolic rates and showing alternative body mass dynamics when rearing an enlarged rather than reduced litter. Those differences in female reproductive effort were nonetheless insufficient to fully match their pups’ energy demand, pups being lighter at weaning in enlarged litters. Interestingly, female reproductive effort changes had long-term consequences, with females that had previously reared an enlarged litter being lighter at birth of their subsequent litter and producing lower quality pups. We discuss the significance of using wild-derived animals in studies of reproductive effort optimisation.