Summary of The Honey-buzzard Movement in 2008

Large numbers of Honey-buzzard Pernis apivorus again migrated through the UK in the autumn of 2008, following the earlier large movement in late September and early October 2000. Unlike in 2000 when careful analysis was required of the weather patterns and Honey-buzzard movements to show that a British origin was more likely, the 2008 movement was much simpler to interpret for the following reasons: the weather patterns were subdued with no unusual events and indeed westerly winds over the southern North Sea at a critical time; the much more detailed information available on the Internet showed a normal Honey-buzzard exit from Scandinavia, mainly keeping well away from North Sea coasts; the better understanding of Honey-buzzard migration strategies, from ringing returns, satellite tracking and studies on drift in Helgoland, indicates a precise SSW or S movement from Scandinavia avoiding sea crossings whenever possible; the fact that two major movements have occurred in Britain in nine years with none in the preceding century suggests statistically that a fundamental change has occurred in the Honey-buzzard populations in Britain. In any event, it is most unlikely that the endangered Swedish population of Honey-buzzard is a source of our migrants as, for most of the 20th century when Swedish populations were much higher, much smaller movements occurred in the UK.

The movement appears to have been of mixed-age birds, exiting from their breeding areas in northern Britain after a highly productive but late season. The gross number of birds, with obvious duplicates removed, recorded in September 2008 is 886. Of these 622 were recorded in the three main regions of East Anglia, NE England and SE England. Numbers in SE England were relatively low. The direction of flight, where noted, was predominantly between S and E (81%) for the UK as a whole and even higher in East Anglia alone at 88%. At no time is there any indication of an influx of birds over the North Sea spreading westerly over the country. Further analysis of the East Anglia data shows that the small number of in-off records (5% of total for the region) may be attributable to birds crossing the Wash or trying a more ambitious sea-crossing further to the east. There is a definite eastward lean to the movement in East Anglia.

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 not known 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 appeared to make a rapid exodus from Benelux through France on favourable N/NW winds as the blocking low pressure over Benelux moved SSE. The birds moved due S, making exceptional speed at 400 km per day, following the movement of the low pressure to the SSE, mainly to the west of the Rhône Valley, arriving east of the Pyrenees from 15/9-16/9. This stream included the birds from Britain: the move over the southern North Sea had paid dividends by enabling a switch to be made away from the Atlantic to more settled weather (and more thermals) on the continent. There appears to have been very little movement SW onto the north-west coast of France. Other birds were noted east of the Pyrenees from 12/9-14/9. These were thought to have taken a relatively direct SW route from Denmark through Germany, moving to the east of Luxembourg and then S towards the eastern Pyrenees. Birds in both movements were drifted by the easterly winds but the ones moving W were slowed down by the blocking low pressure over Benelux. Maps are shown of these movements across the UK and continental Europe.

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.

All the early streams involved a mixture of adults and juveniles. This is true for E Denmark on 11/9 and NE England and Benelux on 13/9 with some adults, albeit a declining proportion, still present on 14/9 in NE England. From 15/9-29/9 only occasional adults were noted in NE England. In SE England where the movement was running at least one day later than further north, juveniles very much predominated with the only adults noted on 14/9. A mixture of juveniles and adult females is suggested by the various photographs.

Looking at the overall totals: the Honey-buzzard exiting Britain (600-800 birds, 13/9-14/9) mainly moved through Benelux (824 birds, 13/9-14/9) before travelling through France to the SW area in the Pyrenees (710 birds, 15/9-20/9). The Honey-buzzard exiting Denmark (1,500 birds, 5/9-11/9) moved initially broadly SW through France to the SW area in the Pyrenees (814 birds, 12/9-14/9) and later broadly S through Italy to Malta (800 birds, 18/9-19/9). The numbers support the mapped routes given earlier but obviously nothing is actually proved.

The Honey-buzzard movement in Britain was remarkable for how long it lasted. From 1/9-12/9 35 migrants were noted, from 15/9-21/9 368 were seen and from 22/9-30/9 98 were noted, giving a monthly total of 1100-1300 birds. With 31 in August and 19 in October, this gives an annual autumn passage of 1150-1350 birds. To estimate the number of breeding pairs it is necessary to know the proportion of juveniles. This is not known but it is safe to say that the total indicates 400-500 pairs (1/3 birds are juveniles, productivity 1 young/pair) up to 750-900 pairs (2/3 birds are juveniles, productivity 1 young/pair) with of course other permutations possible.

So why is it only in occasional years that Honey-buzzard leave northern Britain, following the coast rather than moving inland. Many birds in northern Britain hang on to mid-September to look after the young of the year before departing. By this time thermals are getting weaker and the duration of strong sunshine in the middle of the day very short. The birds therefore may well turn to orographic lift to facilitate their escape. The obvious source of orographic energy is the Pennines, which conveniently runs N-S in the desired direction for the exit. The birds will wait for good conditions of a clearance in the weather on W winds but what if these are not forthcoming. It is believed that 2000 and 2008 were years in which the birds decided they could wait no longer to get away. As the weather cleared to some extent the winds were easterly. It could be argued that the birds could go to the E side of the Pennines for orographic lift but there are a number of reasons why this is not possible. Visibility is often very poor in the Pennines on E winds making navigation difficult. And E winds often do not have the body of W winds making reliance on them for orographic lift more hazardous. So the birds move towards the coast where in Durham and Yorkshire they can get orographic (obstruction) lift from the E winds flowing over the the sea cliffs. There is also a navigational bonus to be obtained from following the line of the coast: the birds will not get disoriented in poor visibility.

Nick Rossiter 2008-2014

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