Multiple elements of the black-billed magpie's tail correlate with variable honest information on quality in different age/sex classes

Animals may be able to assess the quality of other individuals by using information contained in single or multiple traits. We investigated the honesty of the information potentially conveyed through different elements of the tail of the black-billed magpie, Pica pica, which develops in a similar way in both sexes while still in the nest. Variability and age and sex dimorphism were all higher for tail features than for other morphological characters. Tail length of first-year males and females was negatively correlated with the number of feathers with fault bars, which may reflect stressful environmental conditions during feather growth in the nest. Adult males may indicate current body condition through overall tail expression, while the current condition of adult females may be signalled by indicators of stressful environmental conditions during feather growth in the previous year, although both these correlations were weak. Tail length correlated negatively with testes volume of first years, and tails tended to be less damaged in both first-year and adult males with large testes. First-year males that had reached sexual maturity had shorter tails than those that had not. There was no association between parasitism and tail expression, but tail length was positively related to spleen size in first-year males. This study provides evidence that multiple information about individual quality may be conveyed by different features of the tail in ways that vary in different age and sex classes.