Movements in Denmark

It has been suggested (Stuart Reeves, email to UKBN given below) that Falsterbo is an imperfect guide to migration of Honey Buzzards out of Sweden. In particular birds may cross at other points to the north-west of Falsterbo, especially in south-east winds.

It is misleading to regard passage at Falsterbo as the whole story. Karlsson, in Birds at Falsterbo, notes that in south-easterly winds, migrating raptors tend to be drifted more to the North-West, and further that birds migrating down the west coast of Sweden tend to cross to Denmark at the narrowest point, between Helsingborg and Helsingor. In addition Aulen, in Where to watch birds in Scandinavia, notes that autumn totals of Buzzard at Helebaek, Northern Zealand (the part of Denmark nearest to Sweden) often exceed those at Falsterbo. On this basis, the relatively weak passage of Honey Buzzards at Falsterbo may actually indicate that the large majority of the birds migrated by a [much] more westerly route this year.

Stuart Reeves <S.Reeves@marlab.ac.uk>

To examine this possibility, movements of Honey Buzzards in Denmark have been determined using the very detailed observations on web pages available from Skånes Ornitologiska Förening ( http://www.skof.se/lank/andra/aktuella.htm , click on Denmark; or http://www.skof.se/obs/index.html, click on Obsar från världen, followed by Denmark). These observations have been searched for Hvepsevåge for September 2000 and the results are shown from 10th-30th September in Appendix I at the end of this section. Weather observations are included, where given by the observatory, for days when Honey Buzzards were seen and also for days around 20th when the count of Honey Buzzards was zero.

Table 1. Preliminary Results from Analysis of Observations of Honey Buzzards in Denmark from 10th-30th September.

Table 1 summaries the results by day for each part of Denmark. No attempt has been made to remove duplicates. The total of 558 birds in Denmark from 10th-30th September is close to the 638 at Falsterbo over the same period. The agreement is almost certainly spurious to some extent because of the uncertainties surrounding counts at raptor migration points. However, there is a very close parallel in timing between the figures for Denmark and Falsterbo shown in the table. The main difference is that around the time of the relatively heavy passage of mid-September, there is a slight lag in the Danish figures which is entirely as expected. On the 15th it appears the main passage was over Falsterbo with the 153 there being approximately reflected in 118 at Stevns in Denmark. On the 17th numbers were much reduced at Falsterbo but 102 at Klarskov, on the western side of Sjælland near Korsør, could well have arrived by crossing to the north-west of Falsterbo, as suggested by Stuart Reeves.

Very few birds were noted in Denmark or Falsterbo after 17th. The moderate to strong south-east winds did appear to drift about half of the remaining birds to the western side from 19th - 24th. At two well-watched localities in western Jutland (Blåvand, Fanø) a total of 59 birds was seen during this period. The closeness of the two observatories, both near Esbjerg, means there is probably some duplication in these totals. Critically nearly all were following the coast, moving south-east at Fanø and south at Blåvand into the moderate to strong winds (see below). None were moving west. This appears to show the ability of the birds to resist drift out to sea in moderate to strong south-easterly winds.

In addition there was very little indication of birds being drifted west over northern Jutland around 20th. Only one bird was seen in this area, on 21st, and it was moving south. There was also very little indication of birds being drifted west at the German border with just one bird reported in the southern extreme of Jutland. Overall the main picture is that 80% of the birds in Denmark during mid- and late-September were moving down the eastern side of the country following the normal migration pattern to the Rostock area of the Baltic. The last significant concentration on the 17th was about 900km east of north-east England. All were described as juveniles, where the age was determined.

The weather observations in west Jutland in the Esbjerg area are of interest. The mornings at Blåvand at 07:00 were cloudy from 17th-21st with cloud cover ranging from 6/8 to 8/8 but sun was reported at Fanø from 19th-25th. The 17th and 18th were cloudy with drizzle and rain. The wind was a moderate to strong south-easterly from 18th-24th at the exposed Blåvand ranging from 8-12 metres/second (29-43 km/hour). At the apparently more sheltered Fanø the wind was a more gentle 3-9 metres/second (11-32 km/hour). Even at Blåvand, though, the winds on the Beaufort scale ranged from no more than force 4 to 6. At Fanø they ranged from force 2 to 5. Visibility was quite reasonable from 19th-22nd at Blåvand ranging from 25-35km.

The analysis should be repeated when more data are available from other observatories and individuals.

Conclusion for Honey Buzzard: analysis of movements in Denmark, from observations available to date from widespread observatories, appears to confirm that the movements in Scandinavia finished in mid-September. The date of the last significant movement is extended to 17th but the main part of the passage (80%) appeared to be moving, as usual, through the eastern side of Denmark. Only 20% were noted in Jutland, nearly all of which were moving south or south-east against moderate to strong (force 4 to 6) south-east winds.

Other Species: As at Falsterbo, around 20th, Ospreys (Fiskeørn) were scarce at all localities with only occasional singles being noted and Marsh Harriers (Rørhøg) were on the move with several counts of 10-20 in Jutland and Zealand. Common Buzzards (Musvåge) were more frequent in Denmark than at Falsterbo, as suggested above by Stuart Reeves. Their route, like that of the Honey Buzzard, showed a pronounced easterly bias through Denmark with only small numbers in Jutland around 20th. For example the only count reported at Fanø from 13th-26th was 13 moving south-east and 16 resting on 22nd. None were reported from Blåvand in September. In Zealand, however, 610 were seen at Klarskov on 17th and a total of 616 at Stigsnæs and Stevns on 23rd. The Common Buzzard movement peaked one month later at Falsterbo (see Ringing Data for Raptors).

Acknowledgements: many thanks to the Danish bird observatories for making their sightings available on the web and to Skånes Ornitologiska Förening for providing a very convenient entry point; thanks also to Stuart Reeves for suggesting this line of enquiry.

Appendix I: Available observations of Honey Buzzards in Denmark from September 10th-30th 2000

Notes:

Area -- Jylland is called Jutland in Britain and Sjælland is called Zealand.

Direction: trk is short for træk meaning the birds were moving but no direction was specified. R indicates a resting bird.

Wind: unit is metres/second -- 1m/s = 3.6km/hour (unit is explicitly stated on sources, except for Fanø).

Comment: 1-k is juvenile; kl indicates period of observation.

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