THE MIGRATORY TIME PROGRAM OF THE GARDEN WARBLER - IS THERE COMPENSATION FOR INTERRUPTIONS

Fall migratory restlessness (Zugunruhe) of caged first year Garden Warblers Sylvia borin is partly endogenously preprogrammed and it has been suggested that the pattern of Zugunruhe reflects a mechanism that determines the time course and possibly the distance of migration. The question arose of how migrating birds cope with interruptions induced by adverse weather conditions or the need to refuel. Here we have tested the hypothesis that birds compensate for interruptions by adding migratory activity to other nights. An experimental group of 13 Garden Warblers was subjected to repeated schedules of food withdrawal and re-feeding. This treatment resulted in a significant increase in the number of nights without Zugunruhe as compared with the performance of 12 control birds that received food ad libitum. The overall amount of fall Zugunruhe of the experimental birds was reduced correspondingly suggesting that birds did not compensate. This conclusion is corroborated by the finding that those experimental birds in which the treatment induced most interruptions showed the smallest overall amount of Zugunruhe. The results suggest that if a migratory time program determines migratory distance in first year migrants, the wintering area may be relatively variable unless other factors determine its exact position.