Autumn migration - the Direttissima navigation
Newsletter 14/10/2021
The departure of the northern bald ibises to the south is still a long time coming this year. The birds have probably left their breeding grounds everywhere. But they continue to stay on apparently still very abundant feeding areas in the surrounding area. This is not unusual; in previous years, too, the departure has sometimes been delayed until October or even November.
This year, the group of autumn migrants comprises 109 individuals, divided between the breeding areas of Burghausen and Kuchl (53; Bavaria), Überlingen (31; Baden-Württemberg) and Rosegg (25; Carinthia). Of these, 51 individuals are young birds that either grew up in the breeding areas or were released in Rosegg.
The routes of the birds during migration flights vary. But a common pattern can be detected. The birds apparently usually orientate themselves directly to their destination, in autumn the winter area and in spring one of the breeding areas - preferring, so to say, a direttissima navigation. In doing so, the birds apparently perceive very precisely whether and how directly they are moving towards their destination, even at great distances. This direct course is not always possible, especially in the mountains, and of course the current weather conditions can cause a diversion - every pilot knows this only too well.
But an increasing number of individual observations now suggest this interpretation. One recent observation is the somewhat unusual but very exciting flight of the male Akuma from the Überlingen colony, which we can reconstruct using GPS data.
Akuma has spent a longer time in the eastern part of Switzerland and already flew east past the Mont Blanc massif to the south on 18 September this year. From Turin, he single-mindedly followed the direct route towards the winter area, meeting the sea coast in Savona. Usually the northern bald ibises avoid open water areas, but Akuma flew on. The successive GPS positions during the flight over the Mediterranean show in a fascinating way how the bird apparently drifted westwards at first but then was able to adjust its course more and more precisely in relation to the wintering area. After a good 100 km, about a third of the way, it turned back, presumably tired of the constant active flight, as there are no thermals over water.
He didn't fly the same route back to the northwest, but followed a direct course to the north, which, excitingly, led exactly to his breeding grounds in Überlingen - i.e. a direttissima navigation in the "wrong" direction. South of Milan he then paused for several days. From this new position, the direct route towards the winter area again led entirely over land. This was exactly the course Akuma finally followed with determination and reached the WWF protected area Laguna di Orbetello on October 8th. The bird thus provides an impressive example of the precision and flexibility of the navigational skills of these migratory birds.
Image: Extract from Akuma's GPS data in autumn 2021. From Savona, Akuma flew out to the open sea, which is relatively unusual for northern abld ibises. He apparently drifted west at first, but was able to adjust his course more and more precisely towards the winter area. Finally, it flew a direct northward course back to its breeding area. South of Milan, it paused for a few days and finally followed the direct short route to the wintering area.
