ENERGY CONSTRAINTS ON AVIAN DISTRIBUTIONS AND ABUNDANCES

A long-standing hypothesis within ecology states that external environmental factors, such as extreme temperatures, are the primary forces shaping species' biogeographic ranges. On a continentwide scale, the correlative evidence presented in this study strongly indicates that the locations of winter distribution and abundance patterns of several avian species are directly linked with their physiological demands. I calculated the metabolic rate at the northern boundary of the distribution (NBMR) for 14 of 5 1 passerines known to have range boundaries associated with a particular average minimum January temperature isotherm. Based on physiological measures taken from the literature, a regression analysis reveals that the northern boundary metabolic rate (NBMR) is 2.45 times the basal metabolic rate (BMR). The NBMR to BMR ratio shows little interspecific variation; the mean of this ratio is 2.49 with a standard error of ?0.07. Thus, the scalar multiple of BMR ranges from 2.42 to 2.56. This strong association between NBMR and BMR implies that the winter ranges of these 14 birds are restricted to areas where the energy needed to compensate for a colder environment is not greater than 2.5 times the BMR. Body masses of 36 of the remaining 37 birds, those with their range limits associated with isotherms and without physiological measurements, were used to estimate the various physiological parameters. Even when using such crude estimates of these parameters, the approximation of the metabolic rate of individuals at the northern boundaries of their distributions was 2.64 times their BMR. Furthermore, high-density populations of seven of the 14 species with accurately measured physiological parameters were limited to regions where homeostasis requires energy output of no more than 2.13 times their BMR. The range of this scalar extends from 2.08 to 2.34. These findings strongly suggest that on a broad scale the winter ranges of a large number of passerines are limited by energy expen- ditures necessary to compensate for colder ambient temperatures.

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