The field determination of body size and condition in passerines: a report to the British Ringing Committee

This paper assesses the best measure of body size for a range of passerine species, from data provided by British bird observatories. For each species we determine the proportion of variance in body mass explained by wing length, tarsus length, tail length and head length (head + bill) by multiple regression, while also accounting for variance due to two condition components of mass, measured as fat and muscle scores. Across 26 species ranging in mean mas from 5.3 g (Goldcrest) to 100.3 g (Blackbird), fat score was consistently either the best predictor of mass (14 species) or in the set of significant predictors (21 species); muscle score was the best predictor of mass in only one species and a significant predictor in nine. Amongst linear measures, wing length was consistently the best size predictor. The first principal component of size (PC1) based on the four linear measurements frequently explained more variance in mass than wing length alone but the improvement was generally small. Reproducibility of measurements was generally better for wing length than for other linear measures, and in some cases very much better. On the basis of these findings, we recommend the following: wing length is used to give the best general measure of body size within species of passerines; fat and muscle scores, together with total body mass, are recorded to assess individual condition.