A Body Condition Index Based on Body Mass and External Measurements of Live Leach's Storm-petrels

An index of body condition was calculated as the residual from the regression of the cubed root of body mass against the first principle component score of a principle component analysis of six external measurements, which was represented as total body size. This index was called the residual body condition index, or RBCI. To evaluate the utility of the RBCI in studies of Leach's Storm-petrels (Oceanodroma leucorhoa), the RBCI was compared to measurements of total lipid mass. Total lipid content was extracted using ethyl-ether after birds had been killed under anesthesia. The RBCI explained 57.4 % of the variation of the cubed root of total lipid mass. We could thus reasonably estimate the body condition of the parents breeding under natural conditions by measuring their body mass and external measurements.