Basal Metabolism/Thermoneutral Zone

The minimum, or basal, metabolic rate (BMR) for a bird usually occurs when it rests at night and it reaches a maximum during flight. Almost all birds have high basal metabolic rates, and passerine birds have rates 50-60% higher than nonpasserines of the same body size. Basal metabolic rate is related to the size of the animal such that the rate per gram of mass decreases with size. Larger birds use more energy, but they use energy at a lower rate per gram of mass.

Species Mass (gms) Kcal/kg/day Trumpeter Swan 8900 47 Brown Pelican 3500 75 Common Raven 850 108 American Kestrel 110 157 White-crowned Sparrow 27 324 House Wren 11 589 Rufous Hummingbird 3.5 1600

Metabolism and Thermoneutrality In the thermoneutral zone, metabolism and the amount of oxygen used do not change with changing ambient temperature. Metabolism increases below the lower critical temperature (LCT), primarily as a result of shivering heat production. Metabolism increases above the upper critical temperature (UCT) from active loss of heat through panting and evaporative cooling, as well as the direct effects of higher temperatures on cellular functions. The pectoralis muscles are the major source of heat produced by shivering in birds, but in some species is supplemented by shivering in the leg muscles.

Adapted from: Avian Energy Balance & Thermoregulation Gill, Frank B. 1995. Ornithology. Second Edition. W. H. Freeman and Company, NY.