Exciting Behavioural Patterns During Spring Migration
Newsletter 07/04/2021
After a cold front slowed down the Northern Bald Ibis at the end of March, the spring migration is now proceeding all the more rapidly with rising temperatures. Nine birds have arrived in Burghausen, eight in Kuchl and two in Überlingen. Numerous other ibises are on their way north and their flights can be followed on the app Animal Tracker.
For several years now, at the beginning of the breeding season, the two eastern breeding colonies of Burghausen and Kuchl have been joined by an increasing number of birds from the free-flight Northern Bald Ibis colony of the Konrad Lorenz Research Centre Grünau. These birds undergo to a miniature migration from Upper Austria to Salzburg and Bavaria. So far, four ibises from Grünau have arrived in Burghausen and three in Kuchl. This results in mixed breeding pairs in the breeding areas. An exciting situation during the migration season results from that, because the birds from the Konrad Lorenz Research Centre usually return to Grünau in Upper Austria in autumn. Some young birds follow there, while others fly with the rest of the group to Tuscany.
The course of spring migration is usually very precise. The birds follow a direct route towards their breeding areas. Only in the Alps do they deviate from this direct trajectory in order to fly around barriers and find a suitable route over the mountains. For the birds of the two eastern colonies Burghausen and Kuchl, the direct route is in direction N/NE. They cross the Alps on routes through South Tyrol, East Tyrol or Carinthia. Since 2019, an increasing number of ibises have also been migrating to Überlingen on Lake Constance. Their route runs in a N/NW direction and they mostly cross the Alps in the Swiss canton of Valais. Thus, for the first time, two separate migration corridors could be established with the help of human-led migration. This is an important success that also makes it possible to reproduce more complex migration patterns in the context of reintroductions.
A particularly interesting constellation occurs when mixed groups of birds from the western and eastern breeding colonies take off from Tuscany together in spring. This happened on March 26th when two birds from Überlingen (Zoppo and Enea) and two from Burghausen (Adele and Jojo) flew off together. The four birds did not follow either of the two migration corridors but flew straight north, on an intermediate course between the two corridors. Only north of the main Alpine ridge in the Inn valley did the birds separate. From then on, they flew straight to their respective breeding areas. Zoppo has now arrived in Überlingen, Adele and Jojo in Burghausen. Enea is still on the way.
This behaviour, which we already observed in 2019, still surprised us to some extent. It seems that the birds stay together as long as possible and migrate together without deciding between the two flight trajectories. This explains the intermediate course. Only when the directions to the two destinations diverge too far from each other does the group separate and from then on, the birds directly head towards their actual destination. To our knowledge, this peculiarity of migratory behaviour has not been described for any other bird species so far. The special conditions of our project favour such observations.
The behaviour is also a further proof of the ibises' distinctive orientation ability. The intermediate route leads over an unknown area for all the birds involved. But after the separation of the group, the birds seem to correct their course without any problems in order to reach their own breeding area. However, it might also happen that birds arrive in the "wrong" breeding area. Last year, for example, a bird that grew up in Burghausen migrated to Überlingen. We will take a closer look at the circumstances under which this happens. In any case, such mixing is essential to maintain genetic variability in the group.
Picture: Image: Trajectory of the spring migration of Adele (in blue) and Zoppo (in red) to their respective breeding sites compared with the classic migration corridors (in light-blue). Map by Laura Stefani
