This data was the result of an experiment investigating whether biologically relevant differences in reproductive investment influence fitness in a wild bird species. We employed calcium supplementation to experimentally modify reproductive output in Tree Swallow mothers and offspring and we used telomere dynamics during the breeding season as a proxy for fitness. To confirm that calcium supplementation positively impacts reproductive parameters at our study site, we initiated a 4-year trial of calcium supplementation with our Tree Swallow study population. In the following two breeding seasons, we used calcium supplementation to experimentally alter reproductive output and measured telomere length in mothers immediately before egg-laying and shortly before fledging to determine the direct relationship between reproductive investment and telomere shortening in this species. Our study includes one hundred eighty-eight nests initiated and monitored over six breeding seasons. 

This study took place over six summers (2013-2018) at the Colorado State University Mountain Campus located in Larimer County, Colorado, USA (N40.5611, W105.5978). Tree Swallow nests were randomly assigned to either the calcium or the control group and females were captured at the nest. Once in hand, we banded females, took physiological measurements, and classified the birds into age categories of one-year old or greater-than-one-year based on plumage. Also during nest construction in 2017 and 2018, we collected between 10 µl and 30 µl blood samples from females. 

Once a clutch was complete, we measured egg volume and clutch size. After hatching, we measured hatching success for each nest as the proportion of eggs in the clutch that hatched. In 2017 and 2018, blood samples were collected from offspring and mothers when offspring were 12 days old. At this time, chicks were banded, and physiological measurements were taken. All birds were handled and sampled under a Federal Bird Banding permit from the USGS Bird Banding Laboratory and in accordance with approved guidelines of the Institutional Animal Care and Use Committee of Colorado State University (Protocol # 17-7304A).

qPCR was used to measure telomere length relative to the glyceraldehyde-3-phosphate dehydrogenase control gene. To measure change in telomere length of the mother during the breeding season, we calculated D, a measure of temporal telomere shortening adjusted for the regression to the mean. Telomere length measurements at both time-points were transformed to a log-normal scale to avoid negative lengths. 

The datasets can be opened and analyzed in R or Excel.