Early‐Life Telomere Length Negatively Covaries With Brood Size and Father Age, and Predicts Long‐Term Survival in Male Barn Swallows ( Hirundo rustica )

Parental decisions on breeding, such as brood size and timing of reproduction, influence the early-life environment of the offspring, exposing them to variable levels of stressors, which in turn may affect their fitness. In this context, telomeres, conserved DNA sequences at chromosome ends that shorten with age and stress exposure, are relevant because telomere length may predict survival and act as a biomarker reflecting inherited and stress-induced variation in somatic state, thereby linking early-life conditions to future fitness. However, studies examining whether early-life conditions are associated with telomere length and whether telomere length predicts subsequent survival remain limited and provide contrasting results. We examined the effects of parental decisions (brood size and hatching date), as well as parental age and quality (i.e., outermost tail feather length), on telomere length of male barn swallow (Hirundo rustica) nestlings that recruited in their natal site as breeding adults in the following years. We also investigated whether nestling telomere length predicted long-term survival. Telomeres were shorter with increasing brood size, suggesting a detrimental effect of a stressful developmental environment on telomere length. In addition, nestlings raised by older fathers (but not mothers) had shorter telomeres, indicating a possible father aging effect on telomere length. Furthermore, individuals with longer telomeres at the nestling stage showed a higher likelihood of surviving beyond the first year of life. Overall, our results suggest that early-life telomere length may link parental decisions and age to fitness prospects and may thus be a target of selection even in short-lived species.