Counts and Analysis of Migrating Honey Buzzard in Denmark, Sweden, Germany, France, Spain, UK, Belgium, Netherlands and Luxembourg in September 2008

From the Internet, counts were compiled on a daily basis for Denmark, the UK and Benelux. Those for Denmark were assigned to Jutland or the islands to the east, and were additionally classified on the direction of the birds – S/SE/E basically continent-leaning, SW/W some tendency towards the North Sea, træk birds on migration without a direction specified and other (for instance resting). The reasons for obtaining the breakdown was that Denmark is perceived as a potential source of migrants to Britain by some researchers: many Honey Buzzard exit Sweden and Norway via Denmark and if they move to the North Sea side then they may be drifted across the North Sea. The journey due W from western Jutland to Northumberland is about 700km, a very formidable distance for a Honey Buzzard to travel without thermals to assist in their normal economical soar-glide mode. They would have to flap all the way.

Looking at the book Collins Bird of Prey by Benny Génsbøl, published in 2008, we find a section on Autumn Migration in the West Baltic for raptors in general at pp.25-26. Benny lives in Denmark so has first-hand experience. This says:

The autumn migration takes place in a roughly 50km broad corridor between south west Skåne (Falsterbo) and the island of Fehmarn, southwards over the Danish islands. Thereafter the route is southwest towards Hamburg, and then quickly fans out. Each year some 50,000 Common Buzzards and 10,000 Honey Buzzards pass rapidly through East Holstein.

If the prevailing westerly winds switch to the east, the route is diverted westwards and this can result in Honey Buzzards for example being deflected as far as over the North Sea, where they then correct their course by flying back towards the mainland before continuing south.

Two-thirds of the Honey Buzzards migrating over Fehmarn and East Holstein pass through between 26 August and 7 September, and years of observations have shown that most pass the Fehmarn area between 10:00 and 14:00 hours. Honey Buzzards usually travel in small groups.

Common Buzzards migrate somewhat later. A few migrate in early September, but the main migration starts in mid-September, peaking at the end of September and the first week of October. Some 85% pass through Fehmarn and East Holstein between 26 September and 16 October. Mass migration is commoner than for Honey Buzzard, although Common Buzzard also mainly migrate in smaller groups of up to 100 birds.

The second paragraph is very critical: Honey Buzzards actively resist the potential for drift across the North Sea. The birds realise the risks of death or exhaustion are very high in a long sea crossing.

Further information supporting Génsbøl's experience can be found from results obtained at Helgoland, an island in the German Bight, showing that the small numbers of Honey Buzzard occurring in autumn 50km off the German coast move predominantly in directions back to the continent with none moving W or NW, which would be the directions necessary for birds to reach northern England.

Looking at the data below in Table 1 for Denmark we see that 88% of the Honey Buzzard are moving over the islands to the east rather than over Jutland. The main migration finishes on 13th. 63% of the birds migrating over the islands are moving SW so the pattern appears to be normal as summarised by Génsbøl. In Jutland there are much lower numbers of birds moving (about 30 a day from 1st-14th) with just 45% going SW. However, a number of these moving SW are in the southernmost part of Jutland, hence not significantly off route. From 11th-14th just 31 birds were seen in North and West Jutland out of the 163 in Jutland as a whole. So very few birds were on the North Sea side of Jutland from where drift over the North Sea might possibly occur.

The annual Falsterbo figures for the endangered Honey Buzzard in Sweden are very low at 3,415, compared to the long-term average from 1973-2007 of 7,221. The histogram from the Falsterbo pages shows a peak at the end of August and relatively low numbers through September. Only one other raptor shows a significant long-term decline, the Common Buzzard, at 10,166 to date against 14,248 long-term average but the reason for this species is thought to be a change in wintering strategy. Aggregating the day totals for Honey Buzzard for Falsterbo from 15th August-7th October gives 2,462 birds so there is some discrepancy somewhere, which is being investigated. The last significant count is on 9th September of 163 birds. Since 801 birds were in Denmark on 11th (most on Sjælland) it is likely that many moved into Denmark around Helsingør, which is slightly north of Falsterbo, at this time.

The German Bight provides a very appropriate area to study Honey Buzzard drift, as it is here that birds shifted to Jutland by easterly winds might be moved into a position where they drift across the North Sea. The coverage of Germany as a whole by Trektellen is very incomplete but it is better in Schleswig-Holstein on the southern end of the Jutland peninsula. The total for September was 230 with peaks of 115 on 5th, 54 on 11th and 32 on 14th. The maps, on the limited data available, show a slight progression SW on 12th and 13th from northern Germany towards Benelux. On the 14th the birds are moving across the south of Germany. There is no evidence here for movement onto the North Sea coasts. Helgoland, as discussed on the page Experience at Helgoland, is in an excellent position for monitoring drift to the west of Honey Buzzard from the Jutland peninsula. In autumn 2008 there were only two records totalling three birds, with two on 31st August and one on 6th September. This is well below the average in autumn of about 22 per annum found in the more intensive study from 1990-1999.

The figures for Benelux peak sharply on 13th-14th, falling off very rapidly after 15th with only 117 as the total from 15th-30th. Those for Denmark fall off even more rapidly with only 108 seen from 15th-30th. In Benelux the detailed maps available on Trektellen show that numbers throughout are concentrated inland with relatively few birds on the coast. For instance see maps from Trektellen for 12th, 13th and 14th which at first glance show birds moving from the north of the region to the centre and then to the south. The highest counts on the coast are 39 at Parnassia on 13th and 32 at Den Haag on 14th with hardly any over Friesland, which would be a possible staging point for the UK. The maps also show that on each day a number of high counts are very close together, suggesting considerable duplication. Indeed some of the counts at adjacent sites are very similar. While the maps on their own suggest a simple transit through the area, examination of the counts in detail from 12th-14th suggests that the passage through Benelux is a more complex affair. Only 58 birds were noted on 12th mainly in the south-east of the region (with just 8 at Rottumeroog in extreme NE and 7 SW at Vlieland - Pad van Zes in extreme NW); on 13th and 14th much larger numbers appear to the south and west.

The movement through Benelux is reflected very quickly in the figures available for France. It appears that the birds exiting Belgium on the morning of 14th travelled the 800km to the Spanish border, east of the Pyrenees, at speed with 503 noted there on 15th and 124 on 16th with very few thereafter. Conditions were very favourable for a fast crossing of France with strong N/NW winds and clear skies. The birds would have flown around 400km each day, perhaps doing an 8-hour day at 50km/hour. Of course coverage in France is only partial but the lack of passage due south of Belgium after 16th suggests that this source was largely exhausted. See A Full Analysis of the Movement across France for further details.

In Spain the main coverage was near the Straits of Gibraltar. The late movement was not conspicuous here with the last significant passage noted on 12/9 with 451 birds passing Mirador de las águilas (Andalusia) and 1718 noted at Gibraltar (GONHS), probably with some duplication. It is 1068 km from Narbonne in the eastern Pyrenees to Gibraltar, at least 3 days flying under good conditions at 400 km a day. So the movement near Narbonne in France from 12/9-16/9 would not be expected to reach the Straits until 15/9-19/9 at the earliest. From 16/9-21/9 587 birds were counted at the Straits but this is significantly less than the 1441 counted near Narbonne and Perpignan from 12/9-16/9. It is notoriously difficult to produce comprehensive counts at the Straits because of the variety of tactics that are adopted by migrating raptors so it is likely that many birds escaped being counted, when the late wave passed through. Very few were noted at the Straits for the rest of the month, with 147 from 21/9-30/9.

Some additional interesting figures come from Malta where an exceptional count of 700 was made on 19/9.

The figures for the UK are extraordinarily late in their timing. This was also noted for the 2000 movement in the UK. Thus very little movement occurs up to 12th with a total of 35 birds moving but then 411 in the next two days from 13th-14th and in a European context an amazingly high 440 from 15th-30th. It can be seen that during the second weekend of large-scale movements in the UK from 20th-21st, migration in Sweden, Denmark and Benelux had virtually ceased.

Table 1: Provisional Daily and Monthly Totals for Migrating Honey Buzzard in Denmark (Jutland and east), Sweden, Germany, France, Spain, Malta, UK and Benelux in September 2008

Sources:

Denmark DOFbasen

Benelux, Germany, France, Spain Trektellen

Helgoland Orn Arbeits Helgoland

UK BirdGuides

Falsterbo, Sweden SkOF Daily Counts

Gibraltar GONHS 2008 Report

Malta BirdLife-Malta Report (700 count for 19/9 from Sammut, Fenech, & Pirotta (2013); BirdLife recorded 400 this day)

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