The first, large-scale study conducted in the Bay of Biscay on the nocturnal migration of birds has shown that the migration takes place over sea and land in spring, and over land in autumn, contrary to what was once thought. The use of meteorological radars as a method of ornithological observation has been essential in this work, carried out between the Aranzadi Society of Sciences and the UPV/EHU's Department of Applied Mathematics.
Meteorological radars constitute a key tool for studying migratory birds
A study by the UPV/EHU-University of the Basque Country and the Aranzadi Society of Sciences reveals new seasonal patterns in the nocturnal migration of landbirds across the Bay of Biscay
First publication date: 19/04/2017
The research conducted by Nadja Weisshaupt while working on her PhD thesis, and which is ground-breaking in the study of nocturnal migration in the Bay of Biscay, undermines various beliefs: "Migration was thought to take place broadly in autumn, but my study has shown that this is not the case." And the fact is that with respect to migration seasons, the results confirm that high nocturnal activity is recorded in spring and little activity throughout the autumn at Punta Galea. Autumnal migration takes place more towards the Pyrenees possibly to avoid crossing the sea. What is more, lunar observations have made it possible to establish that passeriform birds (the group that includes songbirds) predominate as nocturnal migrating birds, although a small percentage of aquatic birds was also recorded.
The migration of birds involves the journey made by millions of individuals between their breeding areas and the areas they head towards to spend the winter. As the birds migrate, they cover huge distances; to achieve this they have to stop frequently and recuperate so that they can continue on their way. To be able to feed and rest, they need suitable ecosystems in the places they pass through; the absence of suitable environments may have consequences for the population and the conservation of these species. In this scenario knowledge of their migratory strategies is essential, not only from the biological and ecological point of view but also from that of conservation. "To protect a space it is essential to have a scientific basis," explained Nadja Weisshaupt, the author of the study.
The migration of birds has given rise to a whole host of scientific studies, in the Basque Country especially through the ringing of captured birds, in order to analyse the different factors that influence the patterns and dynamics relating to bird migration.
"Yet until now there have been no data on active nocturnal migration in the Bay of Biscay," she added. In order to fill this gap, Weisshaupt has tackled the study of the nocturnal migration of birds on the coast of the Bay of Biscay, and to do so, she used the previous knowledge obtained from Aranzadi on ringing, and the database of the Euskalmet (Basque Met Office) radar on Punta Galea.
Given the problems of night-time visibility, and the need to cover long distances, recourse was made to a range of observation methods: radars, thermal imaging camera and lunar observations.
Meteorological radars are a key tool in this study. Two types of radars were used. One of them was the profiler of Euskalmet (Basque Met Office) located on Punta Galea. The profiler takes wind measurements and during the migration seasons it picks up signals that do not correspond to winds but to birds flying past.
This fact, which poses a major problem for the quality of meteorological data, is useful for studying birds. "This is interesting because we have had to work quite hard to make people believe that it is possible to use profilers in ornithological studies," pointed out Weisshaupt. The second type of radar measures precipitation. The radar on Kapildui, for example, also belongs to Euskalmet. The wavelength of this type of radar is shorter than than of the profiler although it also picks up birds.
But not all radars are suitable; during the study it became apparent that the different meteorological services are subjected to different levels of processing and filtering which means that the available data no longer contain biological information. As a result, the AEMET radars, which would have covered the southern coast of the Bay, were ruled out because only the data from the Euskalmet radar on Kapildui and the French ones contain valid information. With these five remaining radars, an algorithm was used to automate the extracting of data on birds from the meteorological radars; it was developed by Dr Adriaan Dokter of the University of Amsterdam through collaboration carried out within a European ENRAM (European Network for the Radar Surveillance of Animal Movement) COST Action.
The second type of tool, the thermal imaging camera, records infrared radiation emitted by different objects, so it detects "the temperature of a bird, 40-41 ºC, which is in stark contrast to the ambient temperature. The camera allows one to specify whether the birds observed are passeriform ones, whether they are flying alone or in flocks. What is more, the direction and intensity of the migration during the time interval analysed are obtained".
Finally, lunar observation is carried out from two days before until two days after a full moon. "In this case it was used to complement the information obtained from the other systems in order to specify the composition of the group of birds".
This study comes within the framework of the PhD thesis "The role of seas as a geographical barrier for migratory landbirds. An approach to the Bay of Biscay" which the researcher Nadja Weisshaupt (Zürich, 1981) has read at the UPV/EHU. The study was conducted with the collaboration of the Aranzadi Society of Sciences, and the field work was done above all in the Basque Autonomous Community and in part of the Rioja region. The providing of the data from the radars on Punta Galea and Kapildui was made possible thanks to the collaboration of Euskalmet, and the French data were provided by Meteo France. Furthermore, ENRAM COST Action also facilitated two scientific stays abroad.