Energy Control of Size Limits in Birds

Decrease in size is limited by the potential for mobilizing energy Regression lines for standard metabolism and existence metabolism of small passerine birds (< 30 g) at 0⚬ C, where existence metabolism is maximum for many tropical resident and migrant species, intersect at 2.3 g. When allowance is made for additional energy requirements for free existence and reproduction and for adaptations in plumage, behavior, and restriction to warm climates, this correlates well with the weights of the smallest known passerine species of about 5 g. 2. Increase in size is limited by the body's ability to control its temperature at medium to high ambient temperatures. Regression lines for standard metabolism of passerine species in the zone of thermal neutrality and at 0⚬ C intersect at 456 g. The largest passerine birds include species with weights from 450 to over 860 g. Large size birds require adaptations for increased physical regulation of heat loss, restriction to cool climates, or both. 3. Lower rates of standard metabolism in nonpasserine species permit attainment of both lower and greater sizes than in passerme species. 4. The evolution of Passeriformes from nonpasserine ancestors has involved not only a rise in the general level of metabolism but also, as a consequence, a narrowing of the range between maximum and minimum size limits that species can attain. 5. A given biotype will support a larger biomass of large than of small birds but a larger number of individual small birds. Evolution toward small size in birds has occurred, in spite of the greater energy demands involved on the individual and on the biotope, apparently because small size has enabled species to exploit niches which would otherwise remain unoccupied.