ИСТИНА

The notable differences between tropical and temperate birds in numerous life-history and other traits lead to a notion that tropical birds have a “slow pace of life”. Due to trade-offs, most life-history traits can possess only a limited combination of values. The nature of constraints on the diversification of life histories may be related to limitations in physiological mechanisms. Although basal metabolic rate (BMR) is one of the most examined avian physiological traits, the number of studies on energetics in the tropics is very limited. Using the largest existing data-set of metabolic rates of Old World tropical birds (126 individuals belonging to 34 species from 19 families), we studied the relationship between BMR and body mass (M) in wild-caught birds of South Vietnam. The body mass ranged from 5.7 to 150 g. We found that tropical birds have considerably lower BMR than temperate species. The power in the allometric equation BMR=a*M^b was also lower in tropical birds (b=0.586) and significantly different from 2/3 ("Rubner's law"). No difference in BMR was found between passerine and non-passerine species, as well as between oscines ("advanced" passerines) and suboscines ("primitive" passerines). Birds which forage in the sun had BMR’s averaging 15% lower than birds, which forage in the shade. The result is discussed within the context of an energetic model, according to which the high BMR was formed in migrant passerines, which then occupied the forest zone of temperate and high latitudes.