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 passe rine species, from data provided by British bird observatories. For ea ch species we determine the proportion of variance in body mass explai ned 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 speci es); muscle score was the best predictor of mass in only one species a nd a significant predictor in nine. Amongst linear measures, wing leng th was consistently the best size predictor. The first principal compo nent of size (PC1) based on the four linear measurements frequently ex plained more variance in mass than wing length alone but the improveme nt was generally small. Reproducibility of measurements was generally better for wing length than for other linear measures, and in some cas es very much better. On the basis of these findings, we recommend the following: wing length is used to give the best general measure of bod y size within species of passerines; fat and muscle scores, together w ith total body mass, are recorded to assess individual condition.