Analyses of body size evolution in hummingbirds (Trochilidae) are hampered by the lack of standardized weight data for most of the ∼330 described species. Moreover, little is known of the morphological correlates of body weight upon which a body-size index could be constructed. I investigated the skeletal correlates of body weight in the endemic Black-billed Streamertail (Trochilus scitulus) of Jamaica. Mist-netted males were weighed at initial capture and after the holding period (2.1–6.7 hr). Seven of 10 skeletal characters exhibited marginally higher correlations with the second measure of body weight, which was unbiased by gut contents. Sternum length emerged as the single best skeletal predictor of body weight. A multivariate index (PCA 1) derived from three sternal characters had approximately the same predictive value as a comparable analysis of all ten skeletal characters. This suggests that a simple multivariate measure of sternal morphology may provide a reliable body size index for hummingbirds.