Life history theory predicts increased parental investment comes with fitness costs, often expressed as negative effects on survival and future reproduction. To better understand the costs of reproduction and life history trade‐offs, we evaluated calcium supplementation at a high‐elevation site in Colorado as a novel approach to experimentally alter reproductive investment in nesting female Tachycineta bicolor (tree swallow). Calcium is a nutrient critical to avian reproduction as the intake of natural calcium is essential for egg production, embryo development, and nestling growth. Altering calcium availability exclusively during the breeding season allowed examination of individual biological responses to experimental modification of reproduction, as well as the reproductive costs associated with egg production and laying an entire clutch. As a functional endpoint and proxy for fitness and longevity, telomere length was measured at the beginning and end of each breeding season. Telomeres—protective “caps” at the ends of chromosomes—have been shown to shorten with aging and a variety of stressors, including higher reproductive output. Results demonstrate that tree swallow mothers supplemented with calcium during the breeding season experience higher reproductive output and produce offspring with longer telomeres, which came at the cost of relatively shorter telomeres during the reproductive season. These findings provide additional support for reproductive trade‐offs, and also challenge previous calcium supplementation studies that suggest excess calcium reduces the cost of reproduction.
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