3 Counts and Analysis of Migrating Honey-buzzard in NW Europe in September 2008
From the Internet, counts were compiled as far as possible on a daily basis for NW Europe. Sources used were: Denmark DOFbasen, Benelux, Germany Trektellen, Helgoland Orn Arbeits Helgoland, UK BirdGuides, Falsterbo, Sweden was SkOF Daily Counts, no longer extant, now here. The results are shown in Table 8.
3.1 Counts in Denmark
In Section 1 we saw that from the literature Honey-buzzard 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. Below we check that the observations in Denmark in September 2008 are consistent with the literature.
Denmark is therefore of particular importance as it 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. Counts for Denmark were therefore 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).
Looking at the data below in Table 8(a) 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 13/9. 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 1/9-14/9, 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 11/9-14/9 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.
3.2 Counts in other countries in NW Europe
Table 8(b) shows the counts for other countries in NW Europe. For Sweden the annual Falsterbo figures for the endangered Honey-buzzard in Sweden in 2008 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. The last significant count is on 9/9 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. This late movement is discussed later.
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 5/9, 54 on 11/9 and 32 on 14/9. The maps, on the limited data available, show a slight progression SW on 12/9 and 13/9 from northern Germany towards Benelux. On 14/9 the birds are moving across the south of Germany. There is no evidence here for movement onto the North Sea coasts. Helgoland, as discussed above, 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 31/8 and one on 6/9. 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 from 13/9-14/9, falling off very rapidly after 15/9 with only 117 as the total from 15/9-30/9. Indeed 65% of the month's movement occurred on the 2 days 13/9 and 14/9. Those for Denmark fall off even more rapidly with only 108 seen from 15/9-30/9. 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 12/9, 13/9 and 14/9 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 13/9 and 32 at Den Haag on 14/9 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 12/9-14/9 suggests that the passage through Benelux is a more complex affair. Only 58 birds were noted on 12/9 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 13/9 and 14/9 much larger numbers appear to the south and west. The passage through Benelux is considered in more detail below.
It is interesting to consider the figures for NW Europe as a whole including the UK. Table 8(b) shows the totals for the UK as a whole as well as those for Sweden, Germany and Benelux. The remarkable contrast is in the timing within September. Almost as many birds (440, 49.7%) move in the UK in the second part of the month, defined as 15/9-30/9, as in the first part (446). Elsewhere in NW Europe only about 3% of the birds move in the second part, that is only 248 (3.0%) out of the month's total of 8314 in Denmark, Sweden, Germany and Benelux move from 15/9-30/9. This very late passage through the UK is discussed further later.
Migration in NW Europe proceeded as normal in Scandinavia in September 2008 with no sign of exceptional movement onto the North Sea coast of Jutland, with in Denmark 88% of birds moving over the eastern islands. From 11/9-14/9 just 31 birds were seen in North and West Jutland from where movement over the North Sea might start. From eastern Denmark most birds (63%) appeared to move SW as is usual from the literature. The last significant movement in Scandinavia was 801 birds in eastern Denmark on 11/9. In Benelux 65% of the months' movement occurred in 2 days from 13/9-14/9; numbers were concentrated inland but there was an unprecedented movement in coastal areas. In the second part of the month, almost as many birds (49.7%) move in the UK as in the first part; this is very atypical with other countries in NW Europe showing only 3.0% of the birds moving in the second part.