Post-juvenile moult of migrating Ochre-rumped Buntings (Emberiza yessoensis) at an East-Asian stopover site

Abstract The extent and timing of post-juvenile moult depend on various environmental factors. Therefore, information on the post-juvenile moult is key to understanding a species’ migration ecology. We investigated the variation in morphological traits and moult extent of young Ochre-rumped Buntings ( Emberiza yessoensis ) on a stopover island in Korea, as little is known about the moulting strategies of this near-threatened species, which has small, fragmented, and declining populations in East Asia. We found that the extent of post-juvenile moult in Ochre-rumped Buntings was greater than in other migratory Emberiza species. Over 90% of the first-year Ochre-rumped Buntings examined in autumn had at least four primaries and eight rectrices, and about a third of the buntings examined had replaced all their flight feathers. Only two individuals were in active flight feather moult, and the admittedly few spring birds examined had completed flight feather moult, with clear differences in bleaching and wear between inner and more recently replaced outer primaries. This suggests that Ochre-rumped Buntings undergo a complete post-juvenile moult, which is regularly suspended during migration until the late wintering season. During the autumn stopover, we found a positive relationship between body size and the extent of flight feather moult in females, but not in males. The progress of head feather moult observed upon arrival suggests that young birds have mostly completed moulting their head feathers just before likely crossing the Yellow Sea, a major ecological barrier. Our findings corroborate previous observations that Ochre-rumped Buntings have a moult strategy involving a more extensive post-juvenile moult compared to other Emberiza buntings in the region. More research is needed to understand the factors that explain these differences.