Optimal bird migration and predation risk: a field experiment with northern wheatears Oenanthe oenanthe

1. Experiments testing predictions of optimal migration theory so far have concentrated on time and energy as the expenses that birds strive to minimize during migration. Taking advantage of the high variation in numbers of migrating raptors over the island of Helgoland, a field experiment with northern wheatears Oenanthe oenanthe was carried out whereby special attention was paid to predation risk as a possible factor in the organization of migration. 2. Remote weighing of colour-marked birds supplied with ad libitum food revealed a positive correlation between departure fuel load and fuel deposition rate (FDR), an indication that wheatears during autumn migration behave like time-minimizers, which expect global variation of FDR. 3. Predation risk was measured as the rate of raptors flying over. This danger from the environment did not directly affect the departure decision of wheatears, but their FDR was negatively correlated with predation risk. Thus, predation risk influenced the stopover decision via FDR, because time-minimizers are said to skip stopover sites with low FDR when better refuelling conditions may be expected elsewhere along the migration route. 4. Departure fuel load was independent of the cost-benefit relation of predation risk and FDR, a confirmation that wheatears do not directly minimize predation risk during migration.