Complex post-breeding moult strategies in a songbird migrating along the East Asian flyway, the Pallas’s Grasshopper Warbler Locustella certhiola

Moult strategies have received relatively little attention in current ornithology, and knowledge concerning the evolution, variability, and extent of moult is sparse in many bird species. This is especially true for East Asian Locustella species where assumptions on moult patterns are based on incomplete information. We provide evidence indicating a complex post-breeding moult strategy and variable moult extent among Pallas’s Grasshopper Warbler Locustella certhiola, based on data from six ringing sites situated along its flyway from the breeding grounds to the wintering areas. A detailed study revealed for the first time that in most individuals wing feather moult proceeds from the centre both towards the body and the wing-tip, a moult pattern known as partial divergent moult (which is rare among Palaearctic passerines). In the Russian Far East, where both breeding birds and passage migrants occur, a third of the adult birds were moulting in late summer. In Central Siberia, at the northwestern limit of its distribution, adult individuals commenced their primary moult partly divergently and partly with unknown sequence. During migration in Mongolia, only descendantly (i.e. from the body towards the wing-tip) moulting birds were observed, while further south in Korea, Hong Kong and Thailand the proportion of potential eccentric and divergent feather renewal was not identifiable since the renewed feathers were already fully grown as expected. We found an increase in the mean number of moulted primaries during the progress of the autumn migration. Moderate body mass levels and low fat and muscle scores were observed in moulting adult birds, without any remarkable increase in the later season. According to optimality models, we suggest that an extremely short season of high food abundance in tall grass habitats and a largely overland route allow autumn migration with low fuel loads combined with moult-migration in at least a part of the population. This study highlights the importance of further studying moult strategy as well as stopover behaviour decisions and the trade-offs among migratory birds that are now facing a panoply of anthropogenic threats along their flyways.