Findings from Scandinavian Literature on Honey-buzzard Migration
Three sources have so far been consulted as indicated below:
Collins Bird of Prey by Benny Génsbøl
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. Because the ground is flat across the corridor the birds cannot employ orographic lift to facilitate their progress and do drift across the land to some extent.
Orientation System of Honey-buzzard
Map of Honey-buzzard juvenile migration routes from Sweden in Alerstam paper is very revealing. It shows a narrow path moving SSW for the sample of 61 birds, across the base of Denmark, then mainly through Germany and eastern extreme of Netherlands, concentrating to east of Pyrenees and Italy before crossing Mediterranean to Africa. There is no significant westward drift with just one record in the UK and none in western France and nearly all of Iberia. Paper is: Thorup, Kasper, Alerstam, Thomas, Hake, Mikael, & Kjellén, Nils, Can vector summation describe the orientation system of juvenile ospreys and Honey-buzzards? – An analysis of ring recoveries and satellite tracking, OIKOS 103: 350–359 (2003), available from Migration Ecology Group, Lund University, Sweden, here.
From satellite tracking, a second paper on the Lund site confirms the narrowness of the migration routes of adult Honey-buzzard in a SSW direction across western Europe. Juveniles wander a bit more but their direction is more south, rather than tracking W to Gibraltar, making it very unlikely that they will cross the North Sea. Paper is: Hake, M, Kjellén, N, & Alerstam, T, Age-dependent migration strategy in Honey-buzzards Pernis apivorus tracked by satellite - OIKOS 103:341-349 (2003), available here.
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