Moult (the process of feather renewal) takes place at least one time per year among smaller landbirds. Since the protein demand for growing new feathers is high, moult does not usually take place contemporaneously with either breeding or migration. Moult was studied in a migratory landbird, the Pallas’s Grasshopper Warbler Locustella certhiola that breeds across a wide area of north and central Asia and overwinters primarily in South-east Asia. We found that in many birds the process of adult post-breeding moult and renewal of the primaries (the long wing feathers) proceeded divergently from the middle of the primary tract—both outwards towards the wing tip and inwards towards the body—a pattern which is highly unusual among Passeriform landbirds. This post-breeding moult takes place in addition to a second complete pre-breeding moult late in the Palearctic winter, when the birds are still in wintering areas, and before they migrate back to North Asian breeding areas. Moderate body mass levels and low-fat and muscle scores were observed in moulting adult birds. High food abundance in tall grass habitats and a largely overland migration route may enable Pallas’s Grasshopper Warblers to renew avoid laying down large fat reserves prior to migration, further enabling this second (post-breeding) moult.
Eilts, H-J., Feuerbach, N., Round, P. D., Bourski, O., Allcock, J., Leader, P., Davaasuren, B., Erdenechimeg, T., Park, J-G. & Heim, W. 2021. Complex post-breeding moult strategies in a songbird migrating along the East Asian flyway, the Pallas’s Grasshopper Warbler Locustella certhiola: Ecology and Evolution 11: 11–21. https://doi.org/10.1002/ece3.7098