Less hobby in spring?

In September I wrote that some Swedish hobby fly through Libya on autumn passage (see article "Another falcon proven to fly through Libya" dated September 12th). Now new German research suggests that German hobby do so too.

The information is contained in a new paper in British Birds 104, January 2011, 2–15. The paper is called "Two complete migration cycles of an adult Hobby tracked by satellite" and its by Bernd-Ulrich Meyburg, Paul W. Howey, Christiane Meyburg and Klaus Dietrich Fiuczynski.  Bernd-Ulrich Meyburg and colleagues have been carrying out excellent work for some time on tracking birds of prey 

German hobby on autumn and spring passage from paper by Bernd-Ulrich Meyburg et al

However the German tracking research shows that two hobby which flew through Libya on their way south didn't on the spring passage back. They flew back west of Libya.

The paper by Bernd-Ulrich Meyburg et al speculates why this happens. One idea is that they follow their food sources including barn swallow. However I cant find any real evidence that barn swallow fly further west in spring. Indeed there appear to be as many barn swallow in spring as in autumn.

hobby with tracking device by Bernd-Ulrich Meyburg et al

I wonder if they are just following the line of least resistance. There is obviously more vegetation and birds towards the Atlantic coast than inland - something a even a bird can see when flying north but which isn't apparent flying south.

The German research confirms again that hobby fly at night over most of their passage route.

I found two more pieces of information interesting: Hobby fly over Libya during the third or fourth week in September during autumn migration and that females depart slightly earlier than males.

Now we need research on eastern European hobby! Do they  fly further west on spring passage like the German birds or further east up the Nile to avoid a desert crossing?