Evidence for a relationship between bird body size and uncertainty in individual morphological measurements at ringing

Capsule Operator errors when taking morphometric measurements are more important for larger species and for mass measurements.Aims To quantify operator effects on morphometric measurements during ringing sessions, depending on species size and morphometric characters.Methods Three morphometric characters (wing length, body mass and body condition index) had been measured during ringing sessions on 17,818 individuals of 14 different species in southern France. The deviation from the mean and the variation, both absolute and by percentage, of measurements taken by operators were estimated for each bird species and age and sex categories.Results We detected operator effects on measurements. Wing length showed a deviation from the mean, reaching less than 1% of total measurement and a small variation. However, body mass showed a large deviation from the mean, and a large variation reaching up to 10% of total measurement. Body condition index showed broadly the same results as body mass, with the same proportion. Larger species were the most affected by measurement errors, both in absolute and in percentage terms.Conclusion Errors in morphological measurements are common during ringing sessions and are related to the characteristics of the species, particularly their size, the tools used (such as dynamometers for body mass) and the characteristics of the operators.