The Heterophil-to-Lymphocyte (H/L) Ratio Indicates Varying Physiological Characteristics in Nestlings Compared to Adults in a Long-Lived Seabird

Abstract

White blood cells protect the body from microbial threats through two main routes. In birds, heterophils provide an immediate defense by attacking foreign cells directly, while lymphocytes require exposure to foreign antigens to activate cells that are specific in attacking. Under physiological stress, heterophils increase, making the heterophil-to-lymphocyte (H/L) ratio a common indicator of stress in birds. In Kittiwakes, a medium long-lived seabird breeding in the Arctic, we studied the development of these cells in nestlings and compared them to the values in adults. We also tested if the H/L-ratios were correlated to stress or to the body condition. We found that young nestlings (10 days old) in better condition showed an increased H/L-ratio. During growth, from 10 to 25 days of age, the H/L-ratio decreased, reaching the adult level. Adult Kittiwakes in better body condition showed a lower H/L-ratio, the reverse of the pattern in young nestlings. The changes in the adult H/L-ratio were not correlated to the level of stress hormones. Our findings show that, when using the H/L-ratio as a physiological indicator, values must be interpreted with consideration of age, and that the H/L-ratio may not always be a reliable indicator of physiological stress, as commonly assumed.