Altitudinal variation in metabolic parameters of a small Afrotropical bird

Of the numerous factors affecting avian metabolic rate, altitude is one of the least studied. We used mass-flow respirometry to measure resting metabolic rate (RMR), evaporative water loss (EWL) and respiratory exchange ratio (RER) in two populations of a small (10-12g) Afrotropical bird, the Cape White-eye (Zosterops virens), in summer and in winter. In total, 51 freshly wild-caught adult Cape White-eyes were measured overnight. Altitude was included as a source of variation in the best approximating models for body mass, whole-animal RMR, RER, whole-animal standard EWL and whole-animal basal EWL. RER was significantly lower in winter, suggesting a greater proportion of lipid oxidation at lower ambient temperatures (T_a). Cape White-eyes were 0.8g heavier at the higher altitude site and 0.5g heavier in winter, suggesting they may have increased their metabolic machinery to cope with cooler temperatures. EWL was generally significantly lower in winter than in summer, suggesting that birds may increase EWL with increasing T_a, as the need for evaporative cooling increases. Our results support the argument that the subtle and complex effects of altitude (and ambient temperature) should be taken into account in studies on avian metabolic rate.

Of the numerous studies known to affect avian metabolic rate, altitude is one of the least studied. Although trends are not always clear, generally, at higher altitudes, avian metabolic rate increases.

There were statistically significant seasonal and altitudinal differences in various physiological parameters of Cape White-eyes. These results highlight the importance of accounting for altitude in studies of avian metabolic rate.