Using constant-effort catch data, causes of annual variation in the timing of migration of Curlew Sandpipers (Calidris ferruginea) migrating through Ottenby, Sweden, as well as the trend in timing of migration from 1946–2005, was investigated. Variation in the timing of autumn migration of adult and juvenile Curlew Sandpipers was influenced by breeding success connected to predation pressure on the Arctic breeding grounds. Median migration date of adult birds was significantly later in good breeding years compared with poor breeding years while the migration of juveniles was earlier in good breeding years compared with poor breeding years. Also, adults migrated earlier when the average temperature in June was warmer. Median migration dates of adults have advanced by 23 days from 1946–2005, but the migration dates of juveniles have remained unchanged. Unchanged migration dates of juveniles indicate that earlier departure of the adult Curlew Sandpipers from the breeding grounds was not due to earlier breeding. Evidence suggests that declining breeding productivity as a result of increasing predation on broods of shorebirds might, over the years, be the reason for the observed pattern of early departure of adults from the breeding grounds. One possible consequence of earlier migration is a mismatch between timing of migration and periods of food abundance on migration routes and at the wintering grounds, leading to a decline in adult and juvenile survival and population size.
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Vol. 34 • No. 1