Extremely low ambient temperature affects haematological parameters and body condition in wintering Great Tits (Parus major)

In high latitudes, thermoregulatory and behavioural adjustments of birds allow survival under extreme ambient temperatures, changing day length, and food availability. In such habitats with such fluctuating environmental conditions, dominant individuals in social species often monopolise safe microhabitats and food resources, which may lead to greater levels of stress in subordinates. The results of this study revealed that certain haematological indices of health state and body condition of wintering Great Tits (Parus major) were dependent on their sex and age under conditions of extreme ambient temperature. Heterophil and lymphocyte counts revealed a significant increase in heterophil/lymphocyte ratio (H/L) in female Great Tits during the course of cold spell, where the increase was detected mainly in the first year females. We also found that the condition of pectoral muscle during the cold spells deteriorated only in females, especially in the first year individuals. Since sex appears to be the most important predictor of the dominance rank and survival in Great Tit winter groups, elevated physiological stress in adult and first year females during cold spells may be explained in terms of increased resource monopolisation by dominant individuals.