Abstract
Haematological parameters and body mass of four passerine bird species resident in south-eastern Australia were compared in winter and summer to determine whether there were changes indicative of metabolic adjustments associated with seasonal acclimatisation. Haematocrit and whole-blood haemoglobin content did not change seasonally, but erythrocyte numbers increased (by 45-72%) and erythrocyte volume decreased (by 40-73%) significantly in winter in all species. The greater surface area to volume ratio and shorter diffusion pathway of smaller erythrocytes in winter should enhance blood oxygen transport efficiency and thus potentially facilitate metabolic acclimatisation to cold stress. Only one species showed a significant, but small (3%), increase in body mass in winter, suggesting only a minor role for winter fattening in any such seasonal metabolic adjustments in the species studied. The findings contrast with those for some passerines overwintering in the north temperate zone, but the generality of these possible north-south differences needs further investigation.
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