Leucocyte profiles, especially the heterophil/ lymphocyte ratio (H/L), are widely used to assess immune function and stress in birds. In this study, we investigated whether leucocyte profiles in a small colonial seabird, the little auk (Alle alle), are related to age (chicks, adults), sex or biometric traits, including body mass. We blood-sampled chicks at 4-8 and 14-18 days of life and chick-feeding adults with 14-20-and 24-30-day-old chicks. Relative numbers of heterophils and H/L generally differed among chicks and adults, with the highest values in adults with 14-20-day-old chicks and the lowest ones in 4-8-day-old chicks. Females had a higher proportion of lymphocytes than males. The relative numbers of eosinophils were similar in all the groups. The percentage of monocytes tended to differ among the age categories. Analyses of chicks revealed that the percentage of heterophils and the H/L ratios were significantly higher in older than in younger chicks. The opposite pattern was recorded for lymphocytes. In 14-18-day-old chicks, H/L (stress level) was negatively related to body mass, head-bill length and bill width. This time period is energetically stressful as nestlings start to exercise their wings outside the nest chamber. This suggests that the origin of stress in nestlings was mainly nutritional. We found a significant positive correlation between body mass at fledging and the relative number of eosinophils at 14-18 days of life, indicating a higher level of stress in lighter nestlings. The significant relationship between fledging age and the percentage of monocytes at 14-18 days of life suggested a poorer state of health in chicks that fledged at an older age.

In a number of species with biparental care, the parents of one sex desert the mate and brood before the chick is independent. The reasons for this desertion are poorly understood. In the study reported here we investigated whether sex-specific changes in body mass corrected for size (in short body mass) and the stress-related heterophil to lymphocyte ratio (H:L) during the late incubation and chick-rearing period are related to female brood desertion in a small Arctic seabird, the Little Auk (Alle alle). The body mass of both sexes did not change during the study periods, although females were significantly lighter than males. No sex differences were found in the pattern of changes in the H:L ratio. In both sexes, the H:L ratio, and thus the stress level, increased significantly just after hatching and remained high until the last week of chick rearing when it fell back to the level recorded during incubation. These results support behavioural reports of an equal male and female contribution to parental care. The similar body condition of males and females undermines the hypothesis on the depletion of female body reserves as the main trigger of their earlier brood desertion. It is possible that a sex-specific role in parental care during and after fledging and/or an ancestral pattern of parental care are responsible for the transition to paternal care in Little Auks. Keywords

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