Early-life conditions impact fitness, but whether the combined effect of extrinsic stressors is additive or synergistic is not well known. This is a major knowledge gap, because exposure to multiple stressors is likely to be frequent. Telomere dynamics may be instrumental when testing whether combined stressor effects are additive or synergistic, because many factors affect telomere shortening, and telomere shortening predicts survival. We evaluated the effects of manipulated brood size and natural infestation by the carnid fly Carnus hemapterus on nestling growth and telomere shortening of wild jackdaws (Corvus monedula). Telomere length, measured in blood using TRF, shortened on average by 264 bp. In enlarged broods, nestlings’ telomeres shortened more when parasitized, while in reduced broods there was no effect of infestation on telomere shortening. We conclude that there is a synergistic effect of number of siblings and Carnus infestation on telomere shortening rate: blood-sucking parasites may negatively impact telomeres by increasing cell proliferation and/or physiological stress, and coping with infestation may be less successful in enlarged broods with increased sibling competition and lower per capita feeding rate. Larger nestlings had shorter telomeres independent of age, brood manipulation or infestation. Growth was independent of infestation but in enlarged broods, nestlings were lighter at fledging. Our findings indicate that (i) evaluating consequences of early-life environmental conditions in isolation may not yield a full picture due to synergistic effects, and (ii) effects of environmental conditions may be cryptic, e.g. on telomeres, with fitness consequences expressed beyond the temporal framework of the study.

Quantifying an individual’s state as fitness proxy has proven challenging, but accumulating evidence suggests that telomere length and attrition may indicate individual somatic state and success at self-maintenance, respectively. Sexual ornamentation is also thought to signal phenotypic quality, but links between telomeres and sexual ornamentation have been little explored. To address this issue, we examined whether telomere length and dynamics are predicted by the expression of a sexually selected ornament, the length of outermost tail feathers (streamers), using longitudinal data from a population of European barn swallows (Hirundo rustica). We further assessed associations of telomere length with age, sex, breeding status and survival. Telomere length showed high individual repeatability (R = 0.97) across years while shortening with age in both sexes. Telomere length and dynamics were not significantly associated with survival to the next year, remaining lifespan or reproduction status (comparing breeding and non-breeding yearlings). Tail streamer length was negatively associated with telomere length, independent of sex. Thus, telomere length may reflect the costs of carrying an elaborated sexual ornament, although ornament size did not significantly predict telomere shortening. In conclusion, telomere length in adult barn swallows is a highly consistent trait and shows a negative relationship with sexual ornamentation, suggesting a trade-off between sexual ornamentation and telomere length.