Age-specific physiological stress levels in a long-lived seabird

The heterophil-to-lymphocyte (H/L) ratio is widely used as an indicator of physiological stress in birds, yet it remains largely unknown whether H/L ratios vary with age, and, if so, whether such age-specificity reflects within-individual physiological senescence or arises from selective disappearance of birds with certain stress levels. To investigate this, we used 890 blood smears collected between 2019 and 2025 from 485 common terns (Sterna hirundo) of known sex and age. For each smear, we determined the leukocyte composition and calculated the H/L ratio. We found older birds of both sexes to have higher H/L ratios than younger birds, although we found no significant within-individual increase in H/L ratio with age, nor evidence for selective disappearance of birds based on their H/L ratio. Individual repeatability of the H/L ratio across years was very low, but annual variation pronounced and partly explained by colony size. As such, our study suggests variation in H/L ratio to be shaped more strongly by environmental conditions than by stable individual differences, and that senescence may not be underpinned by increases in physiological stress in this species.