Experience at Helgoland with Raptor Drift Migration
Dierschke, V. 2001: Occurrence of raptors on Helgoland (German Bight, North Sea): regular migration or wind drift? Vogelwelt 122: 247 – 256.
Available here.
Abstract (English original, extract):
On the North Sea island of Helgoland (50 km off the German Wadden Sea coast), ten raptor species occur as regular migrants ... High birdwatching activity in the 1990s provided insights into the migration patterns of species which were formerly believed to be rare on the island. However, the occurrence of raptors is strongly influenced by wind conditions. Most raptors were seen while winds were from southeast or east, apparently blowing the birds off the coastline which they usually follow. This effect was most pronounced in the larger species which migrate by soaring, but was less important in smaller species (Sparrowhawk and falcons) migrating by flapping flight. Therefore, larger species seem to reach Helgoland mostly due to wind drift, while especially Sparrowhawk and Merlin seem to cross the German Bight intentionally. Part of the raptors passing the island flew to directions possibly related to correction for wind drift. In autumn this was true especially for the soaring migrants Honey Buzzard and Common Buzzard. No age effect on wind drift was detected as the percentage of immature birds resembled that observed at Falsterbo (Sweden). ... In other species, seasonal patterns of occurrence on Helgoland corresponded to those found at the nearby mainland (northern Germany, Denmark).
Discussion (NR):
The position of Helgoland is shown on the map. It it in a very strategic position to assess wind drift in raptor migration from the Jutland peninsula.
In the paper, Table 1 analyses the wind conditions under which raptors arrive on the island. It is clear that in autumn Honey Buzzard are very strongly associated with E winds so the records do refer to birds drifted from the normal migration routes to the east.
Table 2 is the most significant for the purposes of discussion of whether Honey Buzzard cross the North Sea. It shows 223 Honey Buzzard were recorded in autumn over the ten years 1990-1999, which at about 22 per annum is very low compared to the numbers passing though Denmark but higher than they expected. The flight direction over Helgoland in autumn is very revealing: 83% are steering in a direction to avoid a trip over the North Sea (58% S, 17% SE, 1% E, 5% NE and 1% N; note rounding problems) and only 17% are going SW, which if maintained would bring them to East Anglia. Moving SW, however, does not mean that a North Sea crossing is inevitable by any means. The birds could be looking for landfall on the German coast to the west or in Friesland in Holland. None are going W, which if maintained would land them on the Yorkshire coast. Neither are any going NW, which would be the necessary direction to arrive on the Northumberland coast. It's also interesting that in autumn the percentage of birds identified by the authors as resisting drift is highest for Honey Buzzard (23%) and Common Buzzard (21%).
Comparing the results with those for Sparrowhawk, Kestrel, Osprey and Merlin, which we know cross the North Sea, we see the percentage going SW as much higher at between 71-77% with active resistance to drift lower at 1-12%. Sparrowhawk in particular is quite common as well with about 68 birds per annum in autumn. The Honey Buzzard is the only species of those listed with no W/NW component.
So:
Arrival of Honey Buzzard on Helgoland in autumn is very strongly associated with E winds.
The absolute numbers of raptors in general reaching Helgoland are low
but higher than anticipated before the study started
so some migration (intended and/or involving wind drift) over the North Sea does occur.
The great majority of Honey Buzzard (83%) are steering in a direction, which avoids migrating over the North Sea
with the remainder (17%) moving SW, giving potential landfall in East Anglia but also with the clear option of landfall on the continent to the west
No Honey Buzzard went W/NW, the directions necessary to make landfall in Yorkshire and Northumberland respectively.
Honey Buzzard and Common Buzzard are the two species thought most likely to be resisting wind drift.
Species we know cross the North Sea frequently (from ringing and oil rig records) have a much more marked tendency to move SW (over 71% of birds), were less likely to be avoiding wind drift and also moved W/NW to some extent.
