Univariate Metrics Are Not Adequate to Measure Avian Body Size

In the literature on avian contests and energetics, a single external measure is frequently used to represent overall body size. In an attempt to assess the quality of information available in such external measurements of birds, we measured internal and external elements from museum skeletal plus flat-skin specimens for both sexes of three passerine species. We estimated the "true" overall size of each individual as a factor score computed from the first principal component extracted from a large matrix of skeletal measurements. Bivariate correlations and stepwise regressions indicate that mass or tarsus length, or a principal component factor which combines mass and tarsus length, is the best predictor of overall body size as estimated from bone measurements. Multiple regressions, however, suggest that several external measurements combined often explain only 40-60% of the total variance in overall body size. We suggest that fieldworkers be cautious in their use of single external metrics to represent overall size in small birds. When a single metric for the body size of small passerines is required, fieldworkers should prefer tarsus length or mass to represent overall size.