Females Fly Further-Sex-Specific and Seasonal Differences in Migration Patterns of Pied Avocets

Migration patterns in birds are shaped by ecological and physiological constraints that differ between sexes, seasons and among breeding regions, resulting in complex temporal and spatial strategies. Understanding this variation is crucial for revealing how individuals optimise their annual cycles. As very little is known about the individual migratory behaviour of pied avocets (Recurvirostra avosetta), this study investigated how spatiotemporal migration patterns of avocets breeding in northern Europe vary seasonally, sexually, and spatially. We captured 122 adults on their nests in three countries using walk-in traps and equipped them with lightweight (6-9 g) solar-powered GPS loggers. Avocets spent an extended post-breeding period (on average until early November) in the Wadden Sea before commencing autumn migration. While most individuals followed the East Atlantic Flyway, four deviated via an inland route across the Alps to Italian wintering sites. On average, avocets travelled 1503 ± 996 km within 13.3 ± 19.8 days, resulting in a mean migration speed of 470.8 ± 554.0 km day^-1. The main wintering range was concentrated in France and Portugal, with the Tejo Estuary being the most frequently used site. Migration tactics differed between seasons and sexes: autumn migration lasted longer and involved fewer but extended stopovers, whereas spring migration was faster, with more but shorter stopovers. Females migrated significantly further (1928 km vs. 1277 km in males) and wintered at more southern latitudes, while males remained longer in the Wadden Sea in autumn and tended to arrive at breeding sites earlier in spring. Avocets breeding in the Wadden Sea tended to winter further south than those from the Baltic Sea (Poland), most of which first moved to the Wadden Sea before continuing migration. Our findings highlight the central role of the Wadden Sea as a key fuelling area in the annual cycle of pied avocets from northern Europe.