An investigation into the possibilities of sex and age determination of Eurasian woodcock (Scolopax rusticola L.) based on biometric parameters, using conditional inference trees and minimal important differences

Abstract

Abstract. Morphometric characteristics of Eurasian woodcock collected during spring hunting (March) in Hungary between 2010 and 2014 were investigated to evaluate the accuracy of methods for determining the sex of live birds. We analysed the size dimorphism of biometric traits by sex, age, and sex and age, with sex determination (n = 13,226) performed by destructive methods and age determination based on wing examination (n = 8,905). Using the minimal important differences (MID) method, we demonstrated that, during spring migration, adult females have significantly greater mass and bill length than juvenile females and adult males, as well as a significant difference in body length compared to juvenile females. No biologically relevant differences were demonstrated between the sexes or age classes for other morphometric parameters. Conditional inference trees were applied to test whether body size parameters could be used to separate the age and sex of individuals. Based on posterior probabilities (55.4%), we suggest that biometric parameters no longer provide a sufficiently reliable method to separate age classes during the spring migration. Separation of sexes showed the best results for adult birds, with bill length (85.4%) and body mass (85.2%) proving the best predictors. The inclusion of additional morphometric variables (tarsus, tail, body and wing length) in the model did not increase the reliability of sex segregation, confirming the results obtained using MID, i.e. that there is no statistically verifiable biologically relevant difference between adult male and female birds for these parameters. A methodological innovation in this study was using MIDs for comparisons to determine biological thresholds for differences, the procedure helping to exclude Type I errors and determine biological significance.