Effects of wind and temperature on the migration decisions of a short-distance migratory shorebird across the annual cycle

Elucidating the spatial-temporal characteristics and influencing factors of migration activities is the basis for understanding the life cycle of migratory birds and for making conservation measures. Short-distance migratory birds experience less timing pressure during migration; they tend to migrate under suitable environmental conditions and thus the migration decision might be strongly influenced by external environmental conditions. To test this hypothesis, we deployed GPS tags on Far Eastern Oystercatchers ( Haematopus ostralegus osculans ) in the northern Yellow Sea and tracked their annual migration. The tagged oystercatchers, including five adults and seven immatures (second/third-year birds), mainly wintered along the west coast of the Yellow Sea and bred inland in northeastern China. The migration distance between breeding and wintering sites was 1640.7 ± 260.4 km in spring and 1668.7 ± 306.1 km in autumn. Compared with adults, immatures departed from the wintering sites and arrived at the breeding sites later, while there were no significant differences between adults and immatures in the dates of departure from breeding sites and arrival at wintering sites. Migration initiation generally occurred around sunset. Wind conditions consistently provided tailwinds on departure days from wintering, breeding, and stopover sites during both seasons, with wind assistance exceeding the 5- or 10-day pre-departure average. During autumn migration, the temperature on departure days at stopover sites was lower than the mean temperature over the preceding 5 or 10 days. The Yalu Jiang estuary was the main stopover site during both spring and autumn migrations. The stopover duration in autumn (adults: 118.3 ± 8.4 days; immatures: 130.4 ± 5.1 days) was the longest among the studied shorebirds, likely due to moulting of flight feathers at the stopover site. Our results indicate that the migratory decisions of oystercatchers are strongly influenced by wind conditions during the whole annual cycle, whereas they are also affected by temperature when departing from stopover sites in autumn. The extended stopover at the Yalu Jiang estuary in autumn highlights its importance for the species.