Behavioural and morphological correlates of variation in the extent of postjuvenile moult in the SiskinCarduelis spinus

Abstract

Siskins Carduelis spinus show great variation in the acquisition of adult plumage, so that yearling birds can be classified as either “delayed” (i.e. still showing yearling plumage) or “advanced” (i.e. with an adult plumaged appearance). The extent of moult in males is related to the size of their black bib, which in turn is highly correlated with their social dominance rank. Autumn male body mass is higher in advanced than in delayed moult birds, but by winter the relationship is reversed, suggesting a trade‐off between investment in moult and subsequent body condition. Results were similar in Spain and Britain. A possible cost of advancing moult is suggested by an analysis of aggressive interactions at bird feeding tables which showed that adult males discriminate between delayed and advanced birds, directing most aggression towards adult‐looking yearling males. The results suggest that the variation in the extent of postjuvenile moult is not only related to energetic constraints but has other important behavioural and ecological implications.