Reproductive Drivers of Temporal Telomere Dynamics in Female Tree Swallows

Understanding trade-offs between life-history traits, particularly between reproductive investment and somatic maintenance, remains a major challenge in evolutionary ecology. Telomeres progressively shorten with age and have been suggested as good predictors of survival and longevity. Both intrinsic factors and external stressors, including high reproductive effort, accelerate telomere attrition. Thus, the dynamic nature of telomere length (TL) offers a promising avenue for assessing individual variation in the cost of reproduction. Here, we quantified telomere length (TL) in breeding female Tree swallows (Tachycineta bicolor) from a population breeding in southern Québec, Canada (2023-2024) to assess both cross-sectional and longitudinal changes in TL relative to age, morphological traits, reproductive output, and habitat quality (breeding density). Cross-sectional results showed an age-dependent variation in TL, with evidence of a reproductive trade-off among second year (SY) females with higher fledging success. Longitudinal analyses revealed that earlier breeding in spring was associated with short-term telomere loss, but this trend was not maintained across breeding seasons. In contrast, habitat quality was not associated with telomere changes. Together, our findings support the use of telomere dynamics as a possible biomarker of reproductive effort at both population and individual level. Our results also underscore that the trade-off between reproduction and telomere dynamics is contingent on the temporal scale, specific life history traits, and individual age.