5 Linking the Movements in the UK and Benelux
In September 2008 the very remarkable Honey-buzzard movement was closely monitored in the UK by BirdGuides and in Benelux by Trektellen. In the UK the reaction to the Honey-buzzard movement, which rivals the only other major one in at least the last 100 years (in 2000), has been amazingly restrained, perhaps because it is acutely embarrassing for those in a state of denial on the status of the Honey-buzzard as a breeding bird in Britain. In Benelux it has been very different: that the movement of Honey-buzzard broke a number of day records is obvious from looking at the exuberant comments in column 5 of Appendix I. The movement in Benelux was most noteworthy along the coast and in areas just inland of the coast from Ijsselmeer through to the northern side of the border with France. The movement in Belgium and the Netherlands has been described and discussed by Desmet & Faveyts (2009) http://www.natuurkoepel.be/vwg/archief/roofvogeltrek2008.pdf and their analysis is considered later.
Timings in UK and Benelux
The timing of the movement in East Anglia in the UK is of particular interest as on 13/9-14/9 this is where the relatively large movement 'disappears' with many fewer birds moving into south east England and where there was a definite eastward lean in the direction, as shown above. Table 9 shows a compilation of the timings in East Anglia from BirdGuides. It can be seen that on the 13/9 the movement started early and continued with a peak from 10:00-14:00 BST when 10-20 birds were noted per hour. Movement declined in the afternoon with very little observed after 15:00. A detailed analysis of the timings of the movement through the UK is given above.
Table 9 also contains the timings in Benelux with the region split into a number of geographical areas including the coast and various inland divisions. The compilation of the Benelux data in Appendix I shows which recording points (telposts) are included in each region and gives details of the volume and timing of the movement. The times here are in CEST. Trektellen is organised on a different basis to BirdGuides with daily totals compiled over a specified watch period. This gives advantages in that the weather is recorded, duplicates are less likely at a given station and information on other species is readily available. However, unless provided in the notes for the day, the timing of each bird is not as clear as in BirdGuides. So the timings in Benelux in Table 9 are on a broader basis, either for the whole watch or for a more specific period where the notes enable this to be done. In Table 9 these timings have been converted to BST.
The comparison of the times for the counts in East Anglia and the coastal regions of Benelux on 13/9 indicates that the movement in coastal Benelux was later than that in East Anglia. This is of course very interesting in itself as it is further refutation of the suggestion that the movement in eastern England originated in Benelux. Birds were conspicuous in coastal Benelux in the area of The Hague (Parnassia) from 13:00 BST (14:00 CEST), some three hours after the peak movement started in East Anglia. Table 10 shows some calculations at various flight speeds of how long it would take a Honey-buzzard to cross the North Sea from specified points in East Anglia and Benelux.
Time (hour:min) at 40km/hour
Time at 50km/hour
Time at 60km/hour
Table 10: Crossing Times of the North Sea at Speeds of 40, 50 and 60 km/hour
The soar/glide action is very economical but obviously the overall distance gained per hour is reduced by the time spent soaring in a vertical column. Cranes make 50 km/hour in this mode but harriers only 30 km/hour [Alerstam, 1993, p.265]. Over the open sea soar/glide action is not possible and the birds may opt for the much more tiring power-flight, which however is faster as the soaring phase is omitted. Cranes can glide at 70 km/hour [Alerstam, 1993, p.267]. Honey-buzzards seem to be a little slower than Cranes with Meyer, Spaar & Bruderer  indicating speeds of 37.4 km/hour in soar-glide mode and 49.7 km/hour in horizontal flapping mode. The speed made relative to the ground will of course also depend on the wind. The column for 40km/hour therefore indicates the time taken to travel the distance either in soar-glide mode or with an adverse wind of 10 km/hour in horizontal mode. The column for 50km/hour indicates the time taken in horizontal mode with no wind. The column for 60 km/hour indicates the time taken in horizontal mode with a following wind of 10 km/hour. The weather conditions clearly influence the time taken for the crossing and these are considered next.
The Weather in Britain and Benelux
An overview of the weather for 13/9 September comes from Wetterzentrale. This shows at 00:00 a small low centred over eastern Britain generating the mix of light westerly and variable winds in the southern North Sea noted below. This low tracks SSE over the next two days as shown in the maps from the same source for 14/9 and 15/9. The Honey-buzzards appear to follow the low southwards as the fronts clear. Atlantic weather charts around this period confirm the picture: 11/9 12/9 13/9 14/9 15/9 16/9.
Actual winds at later times in the day can be found by looking at Weather Underground for the first three sources below and Trektellen for the fourth:
For Norwich winds were WSW/W and variable at 1.9-3.7 km/hour from 06:20-08:50 BST including 2 calm spells and variable from 09:20-12:50 BST at 1.9-5.6 km/hour going NNE from 13:20-14:50 BST at 13.0-16.7km/hour (Weather Underground 13/9/2008, parts 1 2 3).
For Southend winds were W/NW from 05:50-06:50 BST at 3.7-11.1 km/hour, calm from 07:50-10:50 BST, WNW at 7.4km/hour at 11:20 BST, calm from 11:50-12:20 BST and N/NW from 12:50-15:50 BST at 7.4-11.1 km/hour including another calm spell at 14:50 BST (Weather Underground 13/09/08, parts 1 2 3).
For Amsterdam winds were NW/N/variable from 06:25-08:55 CEST at 3.7-7.4 km/hour, ENE/E from 09:25-14:55 CEST at 13.0-18.5 km/hour and ENE from 15:25-17:55 CEST at 16.7-24.1 km/hour (Weather Underground 13/9/2008, parts 1 2 3).
For Parnassia near The Hague, winds were NE Beaufort scale 4 (21-29 km/hour) from 14:00-16:30 CEST with the cloud clearing from 8/8 cover to 2/8 through the period (Trektellen 13/9/2008, part 1). Weather conditions at Rotterdam, further S than Amsterdam and closer to Parnassia, were similar to those in Amsterdam (Weather Underground 13/09/08).
So birds moving down the coast of East Anglia at the peak recorded times would have met light and variable winds in Norfolk until around 13:20 when the winds changed to a more decisive but still light NNE wind. Birds moving up to this point may have been encouraged to cross to the continent. Passage slowed markedly at this point. Further down the coast the winds were light westerly or calm for all of the significant period. It is postulated that birds clearing Suffolk, such as the 18 at Minsmere, were tempted by the westerlies to cross over to the continent. However, at some point over the North Sea the birds may well have met headwinds as indicated by the weather at Amsterdam. These headwinds will have slowed the birds down significantly.
It is not possible to quantify the effects of the weather precisely, but if it is assumed that the birds, in horizontal flapping mode throughout, spent half their time with a following wind and half with a headwind, then they will have crossed at a speed relative to the ground of about 50 km/hour and the journey of 160-180 km will have taken 3:10-3:35 hours so birds leaving East Anglia from 10:00-14:00 BST would arrive on the Benelux coast between 14:10-18:35 CEST which is not inconsistent with the observed times of 13:15-15:15 at Parnassia. With a faster speed of 60 km/hour the arrival would be earlier at 13:40-18:00 CEST. The numbers on the coast in Benelux at 74 are very close to those of 83 for East Anglia but not too much should be read into this: counting of raptors on migration is a very difficult science.
The Arrival in Benelux
The comments about the movement at Parnassia, near The Hague, on the Dutch coast are very revealing. On Trektellen it is noted Falsterbo in de duinen !!! (Falsterbo in the dunes!!!'). On an associated web page for news in Naturweg Noord-Holland at 'Telpost Parnassia: nog nooit zoveel trekkende roofvogels op één dag Visarend' [no longer extant] on 15/9 it is reported that on 13/9 at Parnassia:
A record number of birds of prey were counted with a total of 209 seen.
While it is normal for more birds of prey to be seen along the coast in east winds, this movement occurred right at the start of an east wind period.
Probably the raptors had long waited for favourable exit conditions and moved massively as soon as a favourable wind occurred.
The reporters were all perplexed by the movement as it appeared to be the largest in western Europe.
These comments are very perceptive from at least the Honey-buzzard perspective: 1) is self-evident, 2) reflects the situation in that the birds had not come from the east but from the west, 3) has not been documented yet on these pages but it is almost certainly the case, and 4) such a large movement so late in the season cannot be attributed to the usual causes, such as an exodus from Scandinavia.
So if the movement on the coast in Benelux on 13/9 can be explained by the exodus from East Anglia, what about the birds inland in Benelux? The compilation of the Benelux data in Appendix I also gives the numbers and timings of birds in inland areas. Another spectacular movement occurred in northern Holland over Ketelbrug on the eastern side of the Ijsselmeer, where 146 Honey-buzzards migrated on 13/9. Here the movement was earlier than on the coast with 126 moving between 11:00-12:00 CEST as described in the day book for the non-extant site www.ketelbrug.nl/Dagboek%202008/pagina_dagboek_2008.htm. This movement is more of a mystery as there are very few records to the north or east to support the idea that the birds came from this direction, but this potential source does need to be kept in mind. Another possible explanation, looking at the in-off records for Norfolk, is that a determined group of Honey-buzzard moving over Lincolnshire around dawn crossed the Wash on a very eastern course, skirted north-eastern Norfolk and flew over the North Sea, arriving further north than The Hague in the Alkmaar area. Winds were light westerly or variable at this time and there was no easterly component until 09:25 in the Amsterdam area, so in horizontal flapping mode a speed relative to the ground of 60km/hour might be achieved. Table 2 shows this journey would take almost 6 hours, the great majority over the North Sea, so the birds would need to have left at 04:00 BST to arrive in Holland by 11:00 CEST, which seems a very early start. However, such a movement at night may not be unusual for Honey-buzzard. For instance the studies at Malta by Sammut, Fenech, & Pirotta (2013), as discussed later, indicated an exceptionally heavy passage at night around 20/9 just a week later than the movement across Benelux. So it is quite possible for the birds at Ketelbrug to have come from Britain after an early start.
It is also worth considering whether the birds in northern Holland had come from the NE. Numbers crossing at Falsterbo were rather unexceptional with 163 on 9th the last significant count. But Falsterbo is not the only crossing point into Denmark. Indeed some 801 Honey-buzzard were recorded in Denmark on 11th, mainly in Sjælland, having presumably crossed at Helsingør, taking a slightly more northerly route than the normal one at Falsterbo.
The largest count on DOFbasen of 524 was at Skansebakken, Hovedstaden, Nordsjælland, and the report is interesting:
DATA FOR OBSERVATION NR. 6438413 - 11/09/2008
Hvepsevåge (Pernis apivorus) 524
Adfærd: Trækkende mod sydvest
Kommentar til obs.: Det må have været en stor Hvepsevåge dag i Nordsjælland i dag. Jeg kom først på kl.1600. Meget koncentreret træk fra kl.16.00-17.45. De sidste Hvepsevåger kom kl. 18.35 i pæn stor højde. Hvepsevågerne må have mærket kølig luft nordpå ,flere flokke på 80-135 ,de havde virkelig travlt med at komme afsted, og blev ved med at trække i pæn højde, til sent på dagen. Min. 26 1.k. fugle set ,det var dog kun, når nogle kom lavt at de blev pillet ud,så der har formentlig været nogen flere 1. k. fugle.
Lokalitet: Skansebakken (16:00 - 18:45)
Translation through Google is:
DATA FOR OBSERVATION NR. 6438413 - 11/09/2008
Honey-buzzard (Pernis apivorus) 524
Behaviour: Towing southwest
Comment on obs: It must have been a great day Honey-buzzard in North Zealand today. I came first in kl.1600. Highly concentrated drag from 16.00-17.45. The last Honey-buzzards came at. 18.35 in pretty high altitude. Hvepsevågerne [Honey-buzzard] may have noticed cool air north, several flocks of 80 to 135, they were really busy to get away, and continued to pull in a nice height, until late in the day. Min. 26 1.k. birds seen, it was only when some came low that they were picked out, so there was probably no more first k. birds.
Location: fortress hill (16:00 to 18:45)
This records a significant last movement through Scandinavia in 2008 in cooling weather conditions. Most birds were adult apparently with only 26 juveniles (1k) noted. Juveniles do indeed normally fly lower than adults so there is a tendency to overestimate their proportion in migrating flocks. The direction of SW is normal for Nordsjælland in northern Denmark as the birds are aiming for the Lubeck area. Counts on Trektellen for 11/9 indeed do show 52 birds on the northern coast of Germany to the east of Jutland, which is in the expected line of direction of SW from Nordsjælland to Schleswig-Holstein, particularly in easterly winds.
There is little evidence for a movement W from Jutland. Certainly the weather would have permitted such a movement. The winds in Denmark at Copenhagen were E on both 11/9 (Weather Underground 11/9/2008, parts 1 2 3) and 12/9 (Weather Underground 12/9/2008, parts 1 2 3) increasing from 5-15 kph on the former to 20-35 kph on the latter, with dry weather on both days. In Bremen in north-west Germany, winds were E 25-30 kph on 12/9, also in dry weather (Weather Underground 12/9/2008, parts 1 2 3). The weather in north-west Germany on 12/9 was mixed; over the Ruhr winds were light and variable with some rain at Köln (Weather Underground 12/9/2008, parts 1 2 3) but it remained clearer further east, such as at Bielefeld where winds were E 10-25 kph, remaining dry until the evening (Weather Underground 12/9/2008, parts 1 2 3). Birds drifted due W would run into rain as they approached the Benelux border. Maps for 11th-13th on rather limited data on Trektellen show small numbers (13 birds) travelling SW from northern Germany on the eastern side of Jutland towards northern Holland but very few birds on the North Sea coasts themselves. This would then be normal overland migration from Denmark, drifted due W by the easterly winds and forced to a halt by blocking weather fronts on the Benelux/Germany border. On the other hand birds resisting the drift to some extent and moving SW would have had a clearer escape route SSW, avoiding the block on the Benelux/Germany border. It is also likely from the counts at Malta that many of the birds resisted drift altogether and moved S through Italy. So the actual routes taken by the late flock at Nordsjælland are many but the high counts at Malta, 9 days later, suggest that drift to the W was a relatively insignificant route; in addition the low numbers reported in north-east Holland and northern Germany on the western side of Jutland do not indicate that this was a major route.
The Honey-buzzard movement on 13th was also very remarkable inland in central and south Benelux with again a very late movement in the day as shown in Table 9. Here birds arrived in strength from 13:00 BST in the central region and from 15:00 BST in the south. One obvious explanation is that the birds following the clearance of the front in Ketelbrug have moved in turn through the central and south regions and merged with the birds arriving on the coast from East Anglia. It is also likely that some birds from East Anglia have crossed the coast unseen and pressed inland immediately, as the timings of arrival in the inland areas to the south are not inconsistent with the departure times from East Anglia. For instance the 190 km from Harwich-Ghent would take 3 hours 50 min at 50 km/hour so birds departing East Anglia at 12:00 BST would arrive in Ghent at about 16:50 CEST.
The View from Flanders
The English abstract for the paper mentioned above by Desmet & Faveyts (2009) reads:
Top migration over the Low Countries: Unusual numbers of raptors migrated on 13 & 14 September 2008 over the Netherlands and Flanders. They were mainly Honey-buzzards Pernis apivorus, Marsh Harriers Circus aeruginosus and Ospreys Pandion haliaetus. Specific meteorological circumstances, which created a bottleneck in time as well as in space, were the cause of this phenomenon.
In the Introduction the expectations of raptor migration in Flanders are reviewed. The migration corridor for raptors in the flat Flanders landscape extends across a broad front, without straits or mountains to concentrate it. Weather has an effect though. In the spring on strong winds from the E to SE, large numbers of migratory raptors move along the coast. The birds are drifted by the wind to the west pressing them up against the coast. Once there, they avoid ending up over the sea, resulting in their following of the coastline in numbers. It is under such circumstances, that peak days are set at sites such as Zeebrugge or Breskens (Netherlands). Occasionally there are also, in the interior of the Low Countries, top days for raptors (Leysen 2003).
The paper deals in detail with the weather patterns over the continent in Benelux and northern Germany and the southern North Sea from 11/9-14/9. It mentions the numbers over Falsterbo in the first ten days of September: only 842 Honey-buzzard, 170 Marsh Harrier, 3053 Sparrowhawk and 6 Common Buzzard. It then considers that the weather systems over Benelux on 11/9 and 12/9 blocked the birds until the sudden clearance on 13/9 enabled them to continue their movement in spectacular fashion. The narrow corridor (50-75 km wide) of movement of both Honey-buzzard and Marsh Harrier through Benelux is highlighted but no particular explanation is given for the concentration on the coast around The Hague. Pretty-much mixed groups of adult and juvenile Honey-buzzard were noted whereas adult Honey-buzzard had already left western Europe. In Benelux, counts were for Honey-buzzard 285 on 13/9, 539 on 14/9 (together 54% of autumn total for 2008), Marsh Harrier 387, 582 (52%) and Osprey 68, 72 (25%). The largest movement was therefore on 14/9, in time from dawn to shortly after noon. The parallel movement in the UK is mentioned but no attempt is made to analyse it.
In more detail for the blocking effect, on 12/9 raptors trekking S in the extreme north of the Netherlands and the German border run up against a rain front, creating a 'jam' which in fact has built up over several days. So the birds of prey cannot continue because of bad weather above the lowlands. On 13/9 the now weakened rain front remains above the boundary between the northern Netherlands and Germany and retreats N initially, even though driven by the ENE wind. Once the rain front has actually passed S, the conditions suddenly become unexpectedly favourable and the raptors that were stranded for the last few days are all on the wing. This is seen first in the north (see Ketelbrug). Around noon the narrow front giving the rain over the southern Netherlands largely dissolves and the raptors move into the centre and the south of the Netherlands (see Tafelbergheide and Kinderdijk). However, in south-east Belgium a related active part of the same rain-front pushes in at 14:00 into Wallonia. The birds of prey in the northern Netherlands after being stuck for two days see their migration corridor to the south closed again. Their only opening is above Flanders, further to the west. Driven by the east wind they remain therefore, as long as possible on the edge of the rain front to the south. However, by late afternoon the rain spreads into the south of West and East Flanders. All the raptors, along an ever narrower corridor, must return back to the ground. So in the late afternoon the 'stream' has reached the Belgian limit of the movement for the day (see Noordelijk eiland, Anderstad). If we take the route of this concentration of Honey-buzzards and Marsh Harriers, it is striking that the corridor which they employed was actually quite narrow (50 to 75 km), while the rain front was right across the width of the Benelux.
A Possible Pincer Movement in Benelux
The blocking weather patterns identified in northern Holland/Germany from 11/9-12/9 also applied to the eastern parts of the UK. So the spectacular movement on 13/9 in UK could have arisen as well from the end of the blocking weather formation early in the morning, actually from 02:50 in Newcastle after almost continuous, heavy rain from 13:20 on 12/9. Visibility was poor until around 08:00 on 13/9 but some birds may well have started their exit from breeding areas in poor visibility before dawn of 06:35 at say 06:00, when there appeared to be no observers around. Such birds, moving at 55 km/hour could be in north Norfolk a little after noon by doing the 310 km at 55 km/hour in about 6 hours 30 min. Birds departing from Nottingham, for example, at 06:00 could be in north Norfolk at 08:45 by doing the 140 km at 55 km/hour. Birds departing from Scarborough, for example, at 07:00 could be in north Norfolk at 10:40 by doing the 180 km at 55 km/hour. So it is certainly feasible for north Norfolk to be reached by Honey-buzzard in the morning from a variety of places in NE England and the Midlands. These birds formed the stream that poured across the southern North Sea into Benelux. A much smaller stream did not cross the North Sea but continued through SE England towards the Channel.
So it seems that we have the birds in northern Holland arriving from the NE (on the View from Flanders) and the birds on the coast in Benelux arriving from the W (via East Anglia), leading to a possible pincer movement on Holland and Belgium with a rush of late-breeding Honey-buzzard migrant flocks arriving from both the NE (Denmark/northern Germany) and the W (East Anglia). An outline of the pincer movement on 13/9 is shown on the map of the southern North Sea area. These movements would be of very late breeding populations of Honey-buzzard, perhaps from northern Sweden and Finland and from northern Britain.
The concept of a pincer movement is superficially appealing but does not stand up to close scrutiny, at least as a movement of equal force on each front. While there is much support on the ground for a concerted movement of Honey-buzzard eastwards from East Anglia towards Benelux, there is much less evidence from observations on the ground of a movement westwards from Jutland towards Benelux. The pincer movement therefore would represent a larger thrust from the W than the E, as shown on the map. This would fit in with the record numbers of raptors recorded in western Holland and Belgium. Honey-buzzard have been passing through Jutland for centuries and being subjected to strong E winds. Record numbers in Benelux are caused by a new source – Britain – not a change in behaviour on migration from Jutland.
Motivation for Moving E to cross the North Sea
So what was the motivation for crossing the North Sea between East Anglia and Benelux? Possibly the birds were desperate to escape the maritime climate of Britain and get over to the continent as soon as possible to speed up their delayed migration. On 14th the weather improved over the southern North Sea and the birds were clearly able to fly at greater height, judging by the comments on Trektellen. The situation on the morning of 14th was that:
Birds were still passing through East Anglia with quite a number leaning E in direction.
Passage through south east England was relatively subdued.
Relatively large numbers were lifting off the Benelux coast.
Exceptional counts were being made in southern Benelux as the main body of birds moved S.
There thus appeared to be a continuous procession of birds from East Anglia through coastal areas around The Hague to southern Benelux, or directly to the latter. By the afternoon activity was declining markedly as the birds moved S. An analysis is available later of the movement across southern Europe, including France.
Large numbers of Honey Buzzard crossed the southern North Sea on 13/9 from East Anglia to Benelux, first appearing in coastal areas and then moving inland to southern and central areas. At 50 km/hour in flapping mode over the North Sea, the journey of 160-180 km would take 3:10-3:35 hours so birds leaving East Anglia from 10:00-14:00 BST would arrive on the Benelux coast between 14:10-18:35 CEST which is consistent with the observed times of 13:15-15:15 at Parnassia, near The Hague. On 14/9 the movement continued in the morning over East Anglia, coastal areas of Benelux and southern Benelux but the birds moved to greater height as they moved S. From 13th-15th the Honey Buzzards appear to follow SSE the movement of the depression, positioned over eastern Britain on 13th, over western Europe as the fronts clear.
The earliest birds in Holland on 13/9 were noted in the north at Ketelbrug from 11:00-12:00 CEST; this site is some 6 hours flying time from Lincolnshire at 60 kph. It is possible that these birds also came from Britain, after making an early start at say 04:00 GMT after the rain had stopped but well before dawn. Studies in Malta indicate the Honey-buzzard can migrate at night in numbers. It is also possible that the birds in northern Holland came from Jutland in the east. The actual routes taken by a large late flock at Nordsjælland, Denmark, are many but the high counts at Malta, 9 days later, suggest that drift to the W was a relatively insignificant route; in addition the low numbers reported in north-east Holland and northern Germany on the western side of Jutland do not indicate that this was a major route.
The conspicuous movement is attributed to the removal of blocking weather patterns identified in the eastern parts of the UK and northern Holland/Germany from 11/9-12/9. The spectacular movement on 13/9 in UK appears to arise from the end of the blocking weather formation early in the morning, actually from 02:50 in Newcastle after almost continuous, heavy rain from 13:20 on 12/9. Visibility was poor until around 08:00 on 13/9 but some birds may well have started their exit from breeding areas in poor visibility just before dawn of 06:35 at say 06:00, when there appeared to be no observers around. Such birds, moving at 55 km/hour could be in north Norfolk a little after noon by doing the 310 km at 55 km/hour in about 6 hours 30 min. Further examples include birds departing from Nottingham at 06:00 could be in north Norfolk at 08:45 by doing the 140 km at 55 km/hour and those departing Scarborough at 07:00 could be in north Norfolk at 10:40 by doing the 180 km at 55 km/hour. So it is certainly feasible for north Norfolk to be reached by Honey-buzzard in the morning from a variety of places in NE England and the Midlands. These birds formed the stream that poured across the southern North Sea into Benelux. A much smaller stream did not cross the North Sea but continued through SE England towards the Channel.