The heterophil-to-lymphocyte (H/L) ratio indicates varying physiological characteristics in nestlings compared to adults in a long-lived seabird

Abstract

The ratio of the innate heterophils to the acquired lymphocytes (the H/L-ratio) has been advo-cated as a good indicator of physiological stress. Little is known, however, about their develop-ment in wild birds. The present study investigates how leucocyte profiles develop in nestlings of a long-lived seabird, the Black-legged Kittiwake (Rissa tridactyla). We counted blood leucocytes in nestlings of about 10 and 25 days of age (N = 23), and in breeding adults around the time of hatching (N = 40) and calculated the H/L-ratio for all three groups. We further investigated if any variation in the leucocyte counts could be explained by body condition or the stress-related hormone corticosterone. 10-day-old nestlings in better body condition showed greater investment in heterophils, which increased their H/L-ratio. The initial focus on innate immunity shifted to-wards acquired immunity by 25 days, aligning their H/L-ratio with that of adults. In adult Kitti-wakes, however, better body condition correlated with lower H/L-ratio, the reverse of the pattern in young nestlings, and this variation was not linked to stress hormone (corticosterone) levels. Overall, our findings suggest that the H/L-ratio reflect ontogeny of physiological traits and indi-vidual condition. As an indicator, values must be interpreted depending on age, in which its sensitivity may also vary. As such, the H/L-ratio may not always be a reliable indicator of physi-ological stress, in particular related to release of glucocorticoid hormones. ontogeny; leucocytes; Black-legged Kittiwake; stress; corticosterone