The measurement of overall body size in birds

--We compared a number of univariate and multivariate measures of body size used commonly in ornithological research, including eight multivariate measures (from principal components analyses), plus skull length, ulna length, tibiotarsus length, wing length, and weight. Analyses are based on 26 measurements on three randomly selected male and three randomly selected female Savannah Sparrows (Passerculus sandwichensis) from each of 53 different geographic localities throughout the species' range. Six of the eight principal components analyses provided essentially the same information about body size. Analyses based on the variance-covariance matrix of raw or log-transformed data provided first axes that varied most from the other multivariate estimates of size. Among the univariate measures, ulna length, wing length, and body weight contributed information that diverged from the multivariate measures of overall size. Weight better represents general size (i.e. PC I) than wing length, but because of variation in reproductive condition, weight is a far better measure in males than in females. Wing length is not a representative measure of body size. Inasmuch as each principal components analysis provides information about body size on PC I, we encourage researchers to choose among the various approaches according to analytical objectives rather than methodological simplicity or general utility. Received 29 November 1988, accepted 18 May 1989. ORNITHOLOGISTS are frequently faced with the challenge of measuring body size in birds. A measure of overall size is required to test hypotheses predicting patterns of geographic variation (e.g. Bergmann's or Allen's rules; James 1970, Johnston and Selander 1971, Niles 1973, Fleischer and Johnston 1982, Handford 1983, Murphy 1985). An estimate of body size is also required to test hypotheses about the evolution of sexual dimorphism in body size (e.g. Hamilton and Johnston 1978, Johnston and Fleischer 1981, Fleischer and Johnston 1984, McGillivray and Johnston 1987, Rising 1987b). In addition, species must be ranked by body size to test models that predict size ratios among coexisting species in ecological communities (e.g. Ricklefs and Cox 1977, Ricklefs and Travis 1980, Haefner 1981, Sabo and Holmes 1983, Miles and Ricklefs 1984, PullJam 1985, Brown and Maurer 1986, Miles et al. 1987). In physiology, standard measures of metabolic activity are frequently expressed as a function of body size, and it is often useful to examine the relationship of structures or organs relative to overall body size (e.g. Fisher 1947, Kendeigh 1976, Blem 1984, Calder 1984, Paladino 1985, Rising 1987a, Packard and Boardman 1988). Body size, however, is difficult to measure. Perhaps the best measure of overall body size is total mass, but reliable information on mass is often difficult to obtain. Although recent compilations of data contribute much to our knowledge of the mass of birds (Clench and Leberman 1978, Dunning 1984), the available data on mass are all too often unsatisfactory because of seasonal and diet-related variability (e.g. Niles 1973). Consequently, ornithologists commonly use a measure of wing length as an estimate of relative body size (e.g. James 1970, Lack 1971, Snyder and Wiley 1976, Payne 1984, Jehl and Murray 1986, Zink and Remsen 1986). Wing length is measured easily on museum study skins and living birds; however, at least in some cases, it is a poor estimate of body size when compared with other, more precise measurements (Rising 1988). Even discounting measurement error, many factors that are difficult to quantify affect the wing length of a bird. First, wing feathers are subject to wear. Thus, the reliability of measurements of wing length decreases as the feathers progressively become more worn. This may be especially important in studies of sexual dimorphism, because in many species behavioral differences between the sexes lead to sexual differences in rates of feather wear. Second, the wing length of an individual varies from year to year--even though the bird's skeleton is completely ossified (and thus, in this sense, the bird is completely grown). For example, Rising (unpubL data) captured and measured (to the nearest mm) wild Savan666 The Auk 106: 666-674. October 1989 October 1989] Measurement of Body Size 667 nah Sparrows (Passerculus andwichensis) over many years. The average of the differences of measurements of wing length of males captured twice or more during the same year is -0.13 mm (range -4 to +2 mm, n = 32, SE = 0.24), but the average differences in wing length of males captured and measured during more than one year is +1.17 (range -2 to +6 mm, n = 23, SE = 0.43). A sign test shows that within seasons there are as many positive changes as negative changes in the measured wing lengths of individuals. Among years, wing lengths appear to increase more often from year to year than decrease (P = 0.05). Thus, wing length, to some extent, increases with age. Because of the difficulties of obtaining accurate information about body size from wing-length data, people have often used measures of individual bones (e.g. Johnston and Selander 1971) or of organ weight (e.g. Power 1970) as estimates of body size. Alternatively, ornithologists have computed combinations of characters (e.g. the sum of many measures; McGillivray and Johnston 1987, Rising 1987b), or multivariate measures (such as principal component or discriminant function scores; Johnston and Selander 1971, Niles 1973, Zink 1986, Rising 1988) that account for the covariation among characters and extract a "size" axis. There has been considerable discussion concerning which of the models of principal components analysis best extracts a "size component" (Jolicoeur 1963, Mosimann 1970, Mosimann and James 1979, Bookstein et al. 1985, Somers 1986, Rohlf and Bookstein 1987). Here we empirically compare size axes from eight different principal components models and five univariate measures of body size, including mass (weight) and wing length, to determine the relative similarity of these estimates of overall body size.