Delayed autumn migration
Newsletter 15/11/2019
During the last few days winter was coming to Austria, which also affected the Northern Bald Ibises from the breeding areas Burghausen and Kuchl, most of which are still staying north of the Alps. Since the temperatures during the day are usually well above zero and the cold weather front, according to forecasts, soon gives way to milder weather, the birds are not acutely endangered. They are currently in good condition, feeding them is not required.
In itself, the stay of our birds north of the Alps at this time of year is not uncommon. Last year the last birds arrived in Tuscany on the 22nd of November. What is unusual is that our birds did not show any approaches towards the Alps in October 2019, as is otherwise the case from around mid-October onwards as the start to the actual autumn migration. As a result, the birds then cross the Alps in smaller groups and usually fly to Tuscany in a few days. In 2014, our birds showed a similar pattern. At that time they did not start to approach the Alps until December. Ultimately, that was too late to escape the onset of winter in time. We had to catch and transfer the birds, which was still possible with the then small population. Fortunately, the situation is not that dramatic this year. Nevertheless, the delay is dangerous for the birds, especially for the still inexperienced young ones.
It is of great importance for the success of the project to understand the reasons for this delay. However, the control of the autumn migration is one of the most complex behavioural processes in migratory birds. It is an interplay of internal timers, weather conditions and social factors. The knowledge about this interplay is generally very limited.
Usually, the autumn migration in the European Northern Bald Ibis population begins around the 10th of August. At this time, the birds leave the breeding colonies, heading to foraging grounds in the Alpine foothills. This year they mainly foraged on meadows in the municipality of Kuchl south of Salzburg. From October 13th, however, the birds gradually changed location, staying near Salzburg Airport. On the same day, our subadult birds in the wintering area returned from the surrounding feeding areas to the WWF reserve, where they are expected to spend the whole winter (video). We repeatedly observed such cross-regional synchronicities in our birds during the autumn migration period.
From that day on, based on the experiences of the previous years, we expected that the birds would start the actual autumn migration by approaching the Alps. As in 2014, this was not the case, and we assume that this phenomenon is somehow related to the weather conditions. The month of October in 2014 was one of the warmest Octobers in 248 years in Salzburg, with an average temperature of 12.6°C. The daily maximums were on average 5.8°C above the long-term average. In 2019, the temperature average of October was 12.2°C, and the daily maximums were on average 6.4°C above the long-term average (see graph).
The graph shows the deviation of the daily maximum temperature values from the long-term average for October 2014-2019. Exceptionally high temperatures can clearly be seen in the second half of October 2019.
We have to assume that these weather extremes are becoming more common as a result of climate change. This affects the external factors influencing migratory birds and can lead to delays, which in turn can lead to a higher risk and higher loss rates. As Waldrappteam we can’t do anything about it. However, other migratory bird species are apparently coping better with the consequences of climate change during the autumn migration. We therefore assume that other species-specific factors also play a role. In particular, the social structure of our relatively recently reintroduced population differs from a wild population. This will gradually change and we can also partially influence this in order to ensure a more stable migratory behaviour of the Nothern Bald Ibis population in the future.
Currently, we continue to monitor the group and prepare for interventions should they be necessary.
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