Measurements of basal metabolic rate (BMR), body water, fat, and lean dry mass of different organs were obtained in 22 bird species, ranging from 10.8 to 1,253 g body mass. Residuals of BMR (after subtracting BMR allometrically predicted from body mass) were positively correlated with residuals of lean dry heart and kidney mass. Measurements of both BMR and the daily energy expenditure of parent birds (DEEpar) during the period of nestling care as assessed by labeled-water turnover were collected from the literature for 26 altricial bird species. The allometric relationships with body mass in this data set were: log BMR (W) = -1.385 + 0.684 log mass (g) [fraction of variance (r2) = 0.973] and log DEEpar (W) = -0.797 + 0.659 log mass (g) (r2 = 0.967). Residuals of log BMR and log DEEpar were positively correlated with each other. The parallel regressions and correlation of residuals lead to reduced variance in the ratio of BMR/DEEpar (mean 0.301; SD 0.086). We suggest that natural selection has led to an adjustment of the size of organs (such as heart and kidney) involved in sustaining energy metabolism at the DEE maximized during parental care and that size-independent variation in BMR reflects the relative size of this highly metabolically active machinery. These relationships of BMR lead to new interpretations of the decline in mass-specific BMR with increasing body size and decreasing latitude and of the difference in mass-specific BMR between birds and mammals.

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We investigated whether daily 02 consumption (Vo2) could be predicted from heart rate (f,) in five exercising barnacle geese (Branta leucopsis) and compared the accuracy of this method with that of the doubly labeled water (DLW) method. The regressions of Vo2 on f,, based on incremental speed tests, differed among individual birds. The 02 pulse (i. e., i~'o2/f,) progressively increased with exercise level from 0.22 mL 02 heartbeat-' during resting to an estimated 0.47 mL 02 heartbeat-during flight. Daily Vo, was generally underestimated (-3.9.%) by (individual) resting O, pulses but overestimated (+8. 4%) by linear regressions of Vo, on f,. However, it was wellpredicted (+ 0.8%) by the 02 pulses appropriate for each exercise level When using relationships derived from the group of birds, the estimations were generally improved (-3.3% for resting 2O, pulse,-0. 03% for appropriate 02 pulse) but poorer (+13.6%) for the group linear regression. Some of these predictions were better than the estimation of daily CO2 production (Vco2) by the two-compartment model of the DL Wmethod (average algebraic error of+0.9%). We conclude that fH can be used to estimate daily energy expenditure in birds accuratelyprovided that (1) its application is limited to the range of exercise levels in which f, has been calibrated against Vo2 and (2a) ~Vo02-fH relationships are determined for each individual bird or (2b) the fH measurements of severalfree-ranging birds are averaged. Heart rate can also be used to indicate within-day variation in energy expenditure.

The relationship between body size and basal metabolic rate (BMR) in homeotherms has been treated in the literature primarily by comparison between species of mammals or birds. This paper focuses on the intraindividual changes in BMR when body mass (W) varies with different maintenance regimens. BMR varied in individual kestrels in proportion to W 1.67 , which is considerably steeper than the mass exponents for homomorphic change (0.667; Heusner, 1984) for interspecific comparison among all birds (0.677) or raptors (0.678), for interindividual comparison of kestrels on ad libitum maintenance regimens (0.786), and for mass proportionality (1.00). The circadian range of telemetered core temperature also varied more strongly with intraindividual than with interspecific (Aschoff, 1981a) variation in mass. This was due to reduced nocturnal core temperature at low-maintenance regimens, which was, however, insufficient to account for the excessive reduction in BMR. Carcass analysis of eight birds sacrificed revealed a disproportionate reduction in heart and kidney lean mass at low-maintenance regimens. We surmise that variation in BMR primarily reflects variation in these metabolically highly active tissues. This may account for positive correlations found between heart, kidney, and BMR residuals relative to interspecific allometric prediction, and between α and p residuals, as expected on the basis of the constant excess of BMR during α above BMR during p (Aschoff & Pohl, 1970a).In the broad diversity of scientific contributions by Jfrgen Aschoff, the one with greatest impact in animal ecology has been his analysis, jointly with Hermann Pohl, of basal metabolic rates (BMRs) in birds and their dependence on body mass and phase of the circadian cycle (Aschoff and Pohl, 1970a, b). This analysis refined earlier allometric relationships established by Kleiber (1947), Brody (1945), and Lasiewski and Dawson (1967 by demonstrating that BMRs, measured in darkness, at rest, at thermoneutral temperature, and without food being digested, were reduced during the circadian rest phase compared with the active phase. BMRs during the rest phase and their allometric relation with body mass soon became a standard for 1. This paper is dedicated to Professor Jürgen Aschoff, whose breadth of scientific interests and warm intellectual support have become an inexhaustible source of inspiration for us.

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