Genomic projections of (mal)adaptation under future climate change, known as genomic offset, faces limited application due to challenges in validating model predictions. Individuals inhabiting regions with high genomic offset are expected to experience increased levels of physiological stress as a result of climate change, but documenting such stress can be challenging in systems where experimental manipulations are not possible. One increasingly common method for documenting physiological costs associated with stress in individuals is to measure the relative length of telomeres – the repetitive regions on the caps of chromosomes that are known to shorten at faster rates in more adverse conditions. Here we combine models of genomic offsets with measures of telomere shortening in a migratory bird, the yellow warbler (Setophaga petechia), and find a strong correlation between genomic offset, telomere length, and population decline. While further research is needed to fully understand these links, our results support the idea that birds in regions where climate change is happening faster are experiencing more stress and that such negative effects may help explain the observed population declines.

Life history theory predicts increased parental investment comes with fitness costs, often expressed as negative effects on survival and future reproduction. Though annual telomere shortening has been shown to increase with higher reproductive output, we use calcium supplementation as a novel approach to experimentally alter overall reproductive investment and measure telomere shortening exclusively linked to the breeding season. Calcium is a nutrient critical to avian reproduction as the intake of natural calcium is needed for egg production, embryo development, and nestling growth. Altering the amount of calcium availability during the breeding season allows us to examine the individual biological response to the experimental modification of reproduction, and allows us to account for the reproductive costs associated with egg production and laying of the entire clutch. We used calcium supplementation in nesting Tachycineta bicolor (tree swallow) and measured telomere length before and after breeding to better understand the costs of reproduction and life history trade-offs. Measuring telomere length at the beginning and end of each breeding season facilitated evaluation of telomere shortening occurring only during the breeding period of this species. We found that mothers supplemented with calcium had higher reproductive success and greater telomere shortening, though their offspring had longer telomeres at 12 days old. Here we show tree swallow mothers supplemented with calcium had higher reproductive output and offspring with longer telomeres, yet these mothers suffered the cost of shorter expected lifespan, as indicated by relatively more shortening of their telomeres during the reproductive season. Our results provide clear support for previous work on trade-offs between reproduction and longevity and challenge previous calcium supplementation studies that suggest excess calcium reduces the cost of reproduction thereby improving fitness in calcium supplemented parents.