Flight behaviour in migratory songbirds

Advances in tracking technology during the last decade have shown that migratory birds have the capacity to fly longer and faster than we previously thought was possible. Yet, we do not know how birds perform these seemingly impossible travels as it previously only was possible to record spatiotemporal patterns. Now, with the help of new technology, we have the possibility to study not only where birds go, but also how they get there - how high they fly, exactly when they fly and when they stop. The few studies carried out to date using this technology have provided hints of stunning new insights and seriously challenged previously assumed limits on peak flight altitudes, in-flight changes of altitudes, and duration of individual flights. For example, we have discovered a totally unexpected altitudinal behaviour: some bird species change their flight altitude between night and day and fly at extremely high altitudes during the day (up to 6000-8000 m). But what makes a migratory bird fly as high as Mount Everest, even when there are no mountains to cross? And what could this behaviour tell us about the constraints of bird migration?