A Re-Examination of the Relation between Standard Metabolic Rate and Body Weight in Birds

An exponential relation exists between standard energy metabolism and body weight in organisms that is described by the generalized equation: Metabolic Rate = a (Body Weight) b (a) where a and b are empirically derived constants. This equation can be rewritten in the more convenient logarithmic form: log Metabolic Rate = log a + b log Body Weight (b) recognizable as a mathematical expression of a straight line. Hemmingsen (1950, 1960) has reviewed the relation of energy metabolism to body size in all organisms, and argues that a b-value of 0.75 best describes the existing data for unicellular organisms, plants, poikilothermal and homeothermal animals. However, the observed limits of b are 0.63-1.0 among individual groups (Zeuthen, 1953, and others). Despite recent increased interest in avian bioenergetics, a definitive statement concerning the relationship between metabolic rate and body weight in birds has been lacking. Several formulas for this relationship have been presented. Brody and Proctor (1932) fitted the following equation to data on avian body weight and metabolism: log M = log 89 + 0.64 log W (c) where M is in kcal/day and W is in kilograms. This expression, in which the regression coefficient (b) of 0.64 differs markedly from those obtained from mammals (0.73-0.76) by Brody and Proctor (1932), Kleiber (1932, 1947), Benedict (1938), and Brody (1945), has been generally accepted for birds until recently. King and Farner (1961) have commented that “on theoretical grounds there seems to be no reason to believe a priori that the relationship of metabolic rate and body weight should be very different in the homoiotherm classes. ” With many more metabolic values than were available previously, King and Farner re-analyzed the relationship, using more rigorous criteria for including data in their computations. They obtained the following equation: log M = log 74.3 + 0.744 log W * 0.074. (d) King and Farner believe that this equation is superior to that of Brody and Proctor (1932) in predicting the metabolic rates of birds weighing more than 0.1 kg. However, they concluded that it does not adequately portray the metabolism-weight relationship for smaller birds. Equation (d) is statistically indistinguishable from Kleiber’s (1947) equation for mammals, and it is therefore doubtful that the metabolism-weight relationship for birds weighing more than 0.1 kg really differs from that in mammals. King and Farner (1961) discuss the possibility that the avian relationship may be curvilinear in the lower ranges of body weight, since small birds have higher metabolic rates than predicted by their equation. Virtually all of the small birds (< 0.1 kg) are passerines, whereas all but two of the species weighing more than 0.1 kg belong to other orders. Dawson and Lasiewski have suggested (see Lasiewski, 1963 ; Lasiewski et al., 1964) that passerines as a group show the same weight-regression coefficient as nonpasserines, but have a higher metabolism per unit weight than nonpasserines of comparable size. Documentation of this suggestion required additional data on large passerines and small nonpasserines. Now that these are available, it is