ИСТИНА

Obtained results and literature data can be used to formulate a hypothesis about the causes of increase in the basal metabolic rate BMR level during the evolution of the Passeriformes. I propose that high BMR of passerines is associated with their preference for the forest habitats and migration. In these habitats it is disadvantageous to have a high flight speed, which follows from the scaling theory for birds. In order to decrease the flight speed, passerine birds have to change their style of the flight. A new style of flight requires more energy per 1 g of body mass and, as a consequence, a higher BMR. The hypothesis about BMR increase in passerines was suggested based on aerodynamic analysis of the flight speed and characteristics of wing. Allometrical analysis shows that in Passeriformes, flight speed is about 20% lower than in Non-Passerines, which agrees well with the inverted ratio of their of BMR level. The regressions for aerodynamic characteristics of wing show that passerines do not change the morphological characteristics of the wing in order to decrease flight speed. Passerine birds prefer forest habitats. The size range of 5-150 g for birds in forest habitats is almost exclusively occupied by passerines, as a consequence of their extensive energetic capability.