Reproduction is inherently costly. Environmental stressors, such as infection and limited food resources, can compromise investment at each breeding attempt. For example, recent data on captive birds showed that increased reproductive effort accelerates ageing. However, the effects of nutritional status and infection on ageing remain unknown. Telomeres function as protective caps at the ends of eukaryotic chromosomes, and changes in telomere length is a commonly used proxy for ageing. To partially address the mechanisms of ageing following reproduction, we supplemented, medicated or administered a combined treatment to wild blue tits (Cyanistes caeruleus) breeding in central Spain during 2012. The nutritional supplement consisted of two different antioxidants, whereas the medication was an antimalarial treatment against blood parasites. We evaluated the effect of these manipulations on reproductive success and parasite loads in the first breeding season, and on changes in telomere length between two consecutive breeding seasons. Supplemented birds showed no reduction in blood parasite infections in 2012, although they exhibited higher body mass and fledging success. The antimalarial drugs reduced infections by several parasite species, but this had no effect on fitness parameters. In the following season, telomeres from supplemented birds had shortened less. Altogether, we found that supplementation with antioxidants provided fitness benefits in the short term and reduced telomere loss a year following treatment. Our results provide indirect empirical support for accelerated telomere loss as a cost of reproduction.

BackgroundClimate change potentially has important effects on distribution, abundance, transmission and virulence of parasites in wild populations of animals.Methodology/Principal FindingHere we analyzed paired information on 89 parasite populations for 24 species of bird hosts some years ago and again in 2010 with an average interval of 10 years. The parasite taxa included protozoa, feather parasites, diptera, ticks, mites and fleas. We investigated whether change in abundance and prevalence of parasites was related to change in body condition, reproduction and population size of hosts. We conducted analyses based on the entire dataset, but also on a restricted dataset with intervals between study years being 5–15 years. Parasite abundance increased over time when restricting the analyses to datasets with an interval of 5–15 years, with no significant effect of changes in temperature at the time of breeding among study sites. Changes in host body condition and clutch size were related to change in temperature between first and second study year. In addition, changes in clutch size, brood size and body condition of hosts were correlated with change in abundance of parasites. Finally, changes in population size of hosts were not significantly related to changes in abundance of parasites or their prevalence.Conclusions/SignificanceClimate change is associated with a general increase in parasite abundance. Variation in laying date depended on locality and was associated with latitude while body condition of hosts was associated with a change in temperature. Because clutch size, brood size and body condition were associated with change in parasitism, these results suggest that parasites, perhaps mediated through the indirect effects of temperature, may affect fecundity and condition of their hosts. The conclusions were particularly in accordance with predictions when the restricted dataset with intervals of 5–15 years was used, suggesting that short intervals may bias findings.

Early-life environmental conditions can provide a source of individual variation in life-history strategies and senescence patterns. Conditions experienced in early life can be quantified by measuring telomere length, which can act as a biomarker of survival probability. Here, we investigate whether seasonal changes, weather conditions, and group size are associated with early-life and/or early-adulthood telomere length in a wild population of European badgers (Meles meles). We found substantial intraannual changes in telomere length during the first three years of life (both between and within individuals), with shorter telomere lengths from spring to winter and longer telomere lengths over the 2 winter torpor period. In terms of weather conditions, linked to food availability and foraging success, cubs born in warmer, wetter springs with low rainfall variability had longer early-life (<1 year old) telomere lengths. Additionally, cubs born in groups with more cubs did not have significantly shorter early-life telomeres, providing no evidence of resource constraint from cub competition. We also found that our previously documented positive association between early-life telomere length and cub survival probability remained when social and weather variables were included. Finally, after sexual maturity, in early adulthood (i.e. 12-36 months) we found no significant association between same-sex adult group size and telomere length (i.e. no effect of intra-sexual competition). Overall we show that controlling for seasonal effects is important in telomere length analyses, and that badger telomere length functions as a biomarker that reflects the physiological consequences of early-life adversity and subsequent effects on cub survival probability.

Carry-over effects refer to processes that occur in one season and influence fitness in the following. In birds, two costly activities, namely reproduction and moult, are restricted to a small time window, and sometimes overlap. Thus, colour in newly moulted feathers is likely to be affected by the costs of reproduction. Using models of bird vision we investigated male colour change in a free-living population of blue tits (Cyanistes caeruleus) in three sampling occasions: spring 1, winter and spring 2. We related crown, tail, breast and cheek feather colouration after the moult (winter) to the intensity of infections by blood parasites during reproduction (spring 1). In the following spring (spring 2), we explored mating patterns with respect to changes in feather colour (springs 1 vs. 2). Males that were less intensely infected by the malaria parasite Plasmodium while breeding showed purer white cheek feathers in winter, which may indicate higher feather quality. Increased brightness in the white cheek was associated with better body condition during reproduction. In the following season, males with brighter cheeks paired with females that had noticeably brighter cheek patches compared to the male's previous mate. These results suggest that the conditions experienced during reproduction are likely to affect moult and thus feather colouration, at least in the white patch. High quality individuals may allocate resources efficiently during reproduction increasing future reproductive success through variation in mating patterns. Carry-over effects from reproduction might extend not only to the non-breeding phase, but also to the following breeding season.