MIGRATING birds do not seem to be drifted off course by light to moderate unfavourable winds1–4. Birds tend to migrate with tail winds, and various people have proposed that the migrants reduce chances of drift by selecting following winds for their particular goal-directed flights. Some reports, however, support the view that migrating songbirds (passerines) are drifted off course by unfavourable winds5–8. Without adequate knowledge of the goals of the birds of a given flight, both claims are debatable. We have recently obtained, by a new technique, data that clarify the issue.