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Migratory birds face serious time constraints in their life cycles. Species carry on their activities during optimal time windows to maximise their fitness. To understand the temporal patterns of migrating shorebirds, we analysed the pattern of turnover within shorebird communities during northward and southward migration at Chongming Dongtan, a stop-over site in the southern Yellow Sea along the East Asian–Australasian Flyway. Results indicated that temporal patterns in shorebird communities differed between seasons. In the boreal spring, the rate of community turnover was initially slow, then increased rapidly midseason, and finally stabilised. In the boreal autumn, in contrast, the rate of community turnover kept constant. This suggests that shorebird species exhibited more temporal overlap in spring than in autumn, perhaps because time constraints are more severe at breeding than non-breeding grounds. The species sequence was strongly linked with breeding latitude: species that breed farther north occurred at the study site later than those that breed farther south on both northward and southward migration. Moreover, large species were more likely to arrive at the stop-over site early on northward (but not on southward) migration than small species. Integrated with the results from intraspecific studies, we propose that the timing of migration is closely related to the breeding latitude both among and within species.

Selection of timing to match optimal environments is crucial for migrants that breed at high latitudes where there is a narrow time window suitable for breeding. However, birds generally depart from non-breeding grounds in a broad time window. How birds adjust their migration schedule to match optimal timing of arrival at breeding grounds is largely unexplored. We radio-tracked Great Knots Calidris tenuirostris at stopping sites in the southern and northern Yellow Sea during the entire stopping periods to determine their time schedules during northward migration. Great Knots stayed for 40.7 ± 9.2 days (n = 11) in the whole Yellow Sea, with the early arrivals having a longer length of stay than the late arrivals. There was no significant difference in the length of stay between birds that arrived on various dates in the southern Yellow Sea, while the late arrivals spent less time during flights from the southern to the northern Yellow Sea, and stayed for a shorter time than the early arrivals in the northern Yellow Sea. We estimated that the later arrivals can still moult into full breeding plumage and deposit enough fuel for a flight to the breeding grounds before departure. We propose that early and latter migration are the two ends of migratory schedule, with the former adapting to unpredictable and rigorous environments and the latter to stable and favourable environments en route. Stopping sites play an important role for birds to adjust their migration schedule to meet optimal timing of arrival at migratory destination.

We assessed the effects of wind conditions on stopover decisions and fuel stores of migratory shorebirds at Chongming Dongtan in the south Yellow Sea along the East Asian–Australasian Flyway. In spring and autumn, wind directions differed among altitudes and wind speed generally increased with altitude. Numbers of shorebirds were related to wind effects at low altitudes (on the ground and at 300 and 800 m above the ground), wind effects at 300 m being the best predictor of shorebird numbers. In spring, total number of shorebirds and numbers of the four most abundant shorebird species were negatively related to wind assistance at low altitudes, more birds departing when tailwinds prevailed and more arriving when headwinds prevailed. In autumn, however, total number of shorebirds and numbers of the four most abundant species were positively related to wind assistance at low altitudes, more birds departing and more arriving with tailwinds than with headwinds. When tailwinds prevailed, the number of arriving birds was higher than the number of departing birds. The fuel stores of captured shorebirds, represented by their body mass, was related to wind effects and change in wind conditions between two consecutive days in both spring and autumn, captured birds being heavier when headwinds prevailed than in tailwind conditions, and when the wind conditions became less favourable for flight between two consecutive days. Our results suggest that wind conditions affect stopover decisions and fuel stores, and thus the optimal migration and fuel deposition strategies of migratory shorebirds.

Evidence-based protection of migratory birds at flyway levels requires a solid understanding of their use of 'stopping sites' during migration. To characterize the site use of northward-migration great knots Calidris tenuirostris in China, we compared length of stay and fuel deposition during northward migration at areas in the south and the north of the Yellow Sea, a region critical for migrating shorebirds. Radio-tracking showed that at the southern site great knots stayed for only short periods (2.3 ± 1.9 d, n = 40), and bird captures showed that they did not increase their mean body mass while there. In the north birds stayed for 1 month (31.0 ± 13.6 d, n = 22) and almost doubled their mean body mass. Fuel consumption models suggest that great knots departing from the northern Yellow Sea should be able to fly nonstop to the breeding grounds, whereas those from the south would require a refueling stop further north. These results indicate that the study sites in the northern and southern Yellow Sea serve different roles: the southern site acts as a temporary stopover area that enables birds with low fuel stores to make it to main staging areas further north, while the northern site serves as the critical staging site where birds refuel for the next leg of their migration. The rapid turnover rate in the southern Yellow Sea indicates that many more birds use that area than are indicated by peak counts. Differential use of the southern and northern sites indicates that both play crucial roles in the ability of great knots to migrate successfully. © 2013 the Authors. Journal of Avian Biology