Differentiation of animals from different age classes by means of pellet size: Assessment of a field method in European rabbits

Estimating population age structures by faecal pellets sizes is a commonly used field method in some mammal species. We examined the validity of this method in European rabbits based on 1113 pellets from 226 animals with known age, by measuring the intra-individual variation in pellet size and studying the explained variance of calibration curves describing the relation between pellet size and individual age. In addition, we applied a simulation model in order to estimate the accuracy of this method. Pellet size showed a high intra-individual variation and was only moderately correlated with the animals’ age. Modelling revealed that the population age structure assessed by this method deviated considerably from the given structure, indicating a systematic estimation error. We conclude that this method can lead to strongly biased results, restricting its validity. We provide estimation errors, which might be considered if estimates of age structure in wild rabbits populations based on faecal diameters are conducted.