Telomere and plumage dynamics in adult barn swallows
Telomere length, or the change in length, has been linked to fitness, morphological traits, and physiological markers in birds. Plumage color, often implicated in signaling, can reflect individual quality and can change over time within an individual. Understanding the interplay between these two dynamic traits, telomeres and plumage color, and potential confounding factors such as age or sex, could help us better understand whether these traits indicate individual quality or mediate life-history trade-offs. The Individual Quality Hypothesis suggests that both telomeres and plumage color reflect individual quality, so individuals with the preferred plumage trait should have longer telomeres or less attrition, whereas the Costly Telomere Hypothesis states that a trade-off should occur between these traits due to the resource-demanding nature of telomere and plumage color maintenance. From 2019 to 2022, I studied breeding populations of adult barn swallows, measuring their relative telomere length, quantifying their ventral plumage color, and other phenotypic measurements. Telomeres did not shorten with age; however, there was within-individual shortening of telomeres over time. Similarly, plumage color was not predicted by age, but I detected within-individual plumage color darkening over time in both sexes. Furthermore, both telomere length and dynamics varied substantially based on sample year, with the interval from 2019 to 2020 having the most telomere attrition. During the same year interval, change in telomere length was associated with a change in belly brightness in males: the darker the male ventral plumage became, the less telomere attrition they experienced. This relationship supports the Individual Quality Hypothesis. The difference in telomere dynamics across years may be related to environmental conditions; adults in 2020 may have incurred greater costs due to more challenging environmental conditions during the breeding season or their time on the wintering ground. Because telomere dynamics can vary substantially by year, we need more longer-term studies and ones that consider the full annual cycle when possible.