Spring is always an exciting time for UvA-BiTS researchers who study migratory birds as we need to wait for our birds to return from their wintering grounds before we can download our data from the GPS loggers. Until their return, we have no idea which birds are actually still alive. However, on May 7th 2013 an exciting email arrived at the CGE office in Amsterdam. Javier Eloriaga (Tarifa Birding Tours) reported that a male Honey Buzzard carrying an electronic tag had been observed at the Strait of Gibraltar on May 4th by Danish birder and photographer Helge Sørensen (www.birdphotos.dk). As the photographs came in it quickly became apparent that this individual was indeed one of our Dutch Honey Buzzards, making this the first confirmed observation of one of our buzzards outside the Netherlands.

Figure 1: UvA-BiTS Honey Buzzard male arriving in Europe at the Strait of Gibraltar around 8:30 PM on the evening of May 4th 2013 © Helge Sørensen

It seems very logical that this observation occurred at the Gibraltar Strait, as that is one of the main strategic goal-sites for Honey Buzzards migrating between western Europe and Africa. This is because crossing the Mediterranean only takes 14 kilometers there, which is a much safer route considering the risk of fatigue or encountering bad weather during longer sea crossings.

Figure 2: A picture can hardly capture the essence of raptor migration at the southern edge of Europe any better: leaving the Gibraltar Strait behind, this Honey Buzzard is stretching out her wings over Spanish land for the first time in 6 months © Helge Sørensen

In fact, all UvA-BiTS buzzards cross the Mediterranean basin at the Gibraltar Strait every season. However, if you consider that close to 100,000 Honey Buzzards make the crossing at Gibraltar each spring, it is remarkable that one of the currently 10-12 (providing they all survived) tagged Honey Buzzards (of which there are at least 6 with still active devices) was actually picked up there.

Figure 3: The migration from Europe into Africa and back becomes very concentrated near the Gibraltar Strait as demonstrated by these 49 migration tracks collected by 13 individual Honey Buzzards between autumn 2009 and spring 2012. Left and right showing autumn and spring respectively, tracks color coded per individual with symbols indicating years (diamond = 2009 triangle = 2010, square = 2011, cross = 2012). Point locations within each track resampled to hourly steps. Click on image to enlarge.

Based on Helge’s pictures we tentatively identified this bird as the male from a pair breeding in the Dutch forest ‘Drents-Friesche Wold’. Both birds of this pair were tagged for the first time last summer. We are very much looking forward to welcoming these birds back this year (so far 5 others have returned) and to confirm this individual is in fact the bird observed in Spain.

What is especially interesting about visual observations of tagged birds is that they provide useful data to verify (qualitatively) the measurements taken by the GPS device, and that they offer useful context in which to interpret the observed measurements. Luckily, Helge Sørensen is a keen observer who was able to provide a detailed description of his observation, and of the migration around May 4th:

Figure 4: It was 8:30 PM but still our male did not decide to hang around the beach for the night and disappeared with its comrades heading north. Just enough for one more nice shot of our bird in the evening sunlight© Helge Sørensen

“My friend and I stayed in the area of Tarifa between April 27th and May 5th in order to observe the raptor migration. During the first 3-4 days we had westerly winds and we saw very few raptors and only solitary Honey Buzzards. On May 1st – 2nd we had easterly winds but still didn’t see any noticeable migration, probably because we stayed on the east side of Tarifa. However, on (May 3rd) with easterly winds (4 m/s) we moved to the Bolonia area, and already at 9:23 in the morning we saw massive kettels of Honey Buzzards some 10 km inland of Bolonia. At noon we moved out to the coast and the birds were coming in good numbers, usually in groups of 10-30, flying low (20-80m) over the water surface. At around 2 pm we left the area, but came back around 7 PM. In the evening only few birds were coming in, and all of them rather low.

Due to that success on May 3rd we expected May 4th to be another great migration day. We started at the rocks above Bolonia beach at around 9 am. The first 100 or so birds were coming in over the next hour. Then, however, the migration stopped completely. The easterly wind was reaching a velocity of about 5m/s. Until noon, we only saw a handful of birds throughout the day. We were quite puzzled, and guessed that the birds had continued, but perhaps so high that we just couldn’t see them in binoculars or scope. At 7:30 PM few birds began coming in again, most of them arriving low over the sea surface, and gaining height after they met the rocks. We were standing on the first top of the rocks, thereby getting to see the birds at eye-level, or soaring through below us. Probably 100 birds came in until 8:40 pm when the migration stopped.

The UvA-BiTS Honey Buzzard was one in a group of 5-6 birds reaching the coast in active flight (first low over the sea, but we didn’t see him until he came around the ridge of the rock).”

We can only hope that this Honey Buzzard will successfully complete the rest of its journey without hindrance. Ultimately it would prove very interesting if more of these field observations can be collected in the future as these deliver very useful information which is not possible to collect with the GPS loggers. Such observations may lead us to study whether, for example, solitary birds fly differently than those in flocks. We strongly urge all birders out there to keep their eyes peeled for migrating Honey Buzzards (and other species) carrying transmission or logging devices and to record such observations with pictures, film or field notes in as much detail as possible.