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.

Parental investment comes with increased fitness costs, often expressed as negative effects on survival and future reproduction. We used a novel approach to experimentally alter reproductive investment in a wild bird population and measured telomere length before and after breeding to better understand the costs of reproduction and life history trade-offs. Telomeres are terminal features of chromosomes consisting of highly conserved DNA repeats that shorten with age and stress and whose length is positively correlated with lifespan. We assessed the effects of calcium supplementation on reproductive parameters and telomere length in Tree Swallows (Tachycineta bicolor) in northern Colorado. Measuring telomere length at the beginning and end of each breeding season facilitated evaluation of changes exclusively caused by maternal investment in reproduction. We found that Tree Swallows supplemented with calcium had higher reproductive success and greater telomere shortening compared to control birds. Although mothers supplemented with calcium suffered increased telomere attrition, offspring in calcium supplemented nests had longer telomeres at 12 days old. Thus, Tree Swallow mothers supplemented with calcium had higher reproductive output and offspring with longer telomeres yet these mothers suffered the cost of lower expected maternal lifespan, as indicated by shorter telomeres during the reproductive season.