How do common swifts cross the Sahara?
We have been able to record the Sahara crossing by 72 Scandinavian common swifts by GLS (so-called light-loggers). Common swifts cross the Sahara by longer routes and slower speeds (average migration speeds 190-235 km/day, average crossing time 22-30 days) in autumn as compared to in spring (average migration speeds 750-900 km/day, average crossing time 5.5 days). The swifts migrated across the western part of the Sahara on broad front in autumn, while they followed three major flyways across the Sahara in spring. A western route was preferred by most Scandinavian common swifts in spring, including a stopover period in Liberia before the Sahara crossing was initiated, while the rest of the swifts crossed the Sahara by central or eastern routes, the latter crossing the Arabian Peninsula. The swifts selected to migrate in tailwind conditions and were spending time at stopovers in autumn (southern Europe and North Africa) as well as in spring suggesting fueling before the Sahara crossing.
The preferred western flyway cross the Sahara in spring was associated with good wind conditions and time spent at stop-over, suggesting it has evolved as a response to a combination of timing availability of aerial insects (swarming termites in April and May) and tailwinds facilitating the barrier crossing.