Determinants of primary moult suspension in Dunlins (Calidris alpina), migrating through the southern Baltic coast

Moult in birds is a highly energetically expensive process. Therefore, feather replacement in most species does not coincide with breeding or migration periods. However, when time constraints prevent completion before migration, many birds suspend feather replacement and resume it post-migration. One species exhibiting active primary moult during migration is a small wader–the Dunlin ( Calidris alpina ). Despite being one of the most extensively studied of their group in the western Palearctic, some aspects of its biology, such as primary moult suspension and factors influencing it, remain poorly understood. We investigated the occurrence and determinants of primary moult suspension in Dunlins during autumn migration along the Gulf of Gdańsk coast between 1985 and 2023. Our results revealed that only a small proportion (14%) of primary moulting individuals suspended the process, though annual variability was noticeable. In most years, the capture date significantly influenced moult suspension frequency, with later-captured birds more likely to halt the process. Moult suspension was more frequent in older birds than second-year individuals, males than females, and individuals with a higher body condition index. These patterns align with known interpopulation variation in moult strategies and sex- or age-related differences in breeding investment. However, the trends were inconsistent across years, potentially reflecting annual variability in breeding costs across age and sex classes. As a highly energy-demanding process, primary moult is likely influenced by cumulative energy expenditure during breeding, highlighting trade-offs between reproduction and self-maintenance in a migratory species. Therefore, suspension of the primary moult during migration in Dunlins might occur more frequently in individuals more strongly engaged in breeding and parental care, and birds that halt the process tend to be in better overall body condition.