6 Ages of the Honey-buzzard
A very important question is: what was the age of the birds at the start of the movement? We do have some information in Appendix II: Regional Reports and other Original Sources for the Honey-buzzard Movement, from which the summaries below are extracted but the data is incomplete in the sense that many birds seen were unaged:
Quite a lot of adult specimens were observed, in mixed groups with juvenile birds, in Benelux. See for instance Geitenbos on 14/9 where 11 adults and 7 juveniles seen in total of 46 birds. Adults had already largely withdrawn from Western Europe by the time of the movement (Desmet & Faveyts (2009)).
In the last significant movement through Scandinavia at Skansebakken, Nordsjælland, Denmark, on 11/9, 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.
In Northumberland 5 migrants were seen making an exit from their Tyne Valley breeding area on 13/9. All were adult: 1 male, 4 female, with juveniles soaring but not making the exit. Visits to the SW Northumberland Study Area revealed totals from 1/9-7/9 of 28 birds (6 male, 5 female, 17 juvenile). It had been a very productive breeding season with 35 pairs raising 52 young.
In ageing of migrants in NE England, the Northumberland, Durham and Cleveland Reports give a total of 9 adult, 25 juvenile and 15 unaged on 13/9 (26% adult, excluding unaged); 2 adult, 11 juvenile and 9 unaged on 14/9 (15%); 2 adult, 20 juvenile and 8 unaged from 15/9-24/9 (9%).
In ageing of migrants in SE England, the London and Sussex Reports give a total of 0 adult, 1 juvenile and 0 unaged on 13/9 (0%); 2 adult, 8 juvenile and 5 unaged on 14/9 (20%); 0 adult, 8 juvenile and 11 unaged from 15/9-29/9 (0%).
Very little ageing information is provided in county reports or on BirdGuides for migrants in East Anglia.
Overall it does seem clear that 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. Distinguishing adult male Honey-buzzard from females and juveniles is relatively easy but more experience is needed to separate adult female and juveniles. Indeed a mixture of juveniles and adult females is suggested by the various photographs:
North East Scotland Bird Report 2008. The picture shown on the plates between p.80-81 of a Honey-buzzard at Ythan (12/9 or 17/9) is not a juvenile as claimed in the text. It's an adult female with dark bill, evenly spaced bars across remiges, 3 bars on tail and black on wingtips restricted to the fingers. It seems unbelievable that an adult Honey-buzzard would cross the northern North Sea from Norway to Aberdeen.
Lees (2009). The paper does include 2 plates of Honey-buzzard taken in Lincolnshire, one at Frampton Marsh (presumed 14/9) and the other at Gibraltar Point (dated 14/9). Neither is aged. The one at Frampton Marsh appears to be a juvenile with pale bill and an extensive dark area on the wingtip. The one at Gibraltar Point is certainly a juvenile with pale bill and an extensive dark area on the wingtip.
Birds in Northumbria 2008. A photograph of a juvenile on 13/9 at Earsdon confirms the ageing with yellow bill, extensive black on wingtips and 4 bars across remiges, including subterminal band.
Cambridgeshire Bird Report 2008. A photograph of a dark-phase juvenile at Fen Drayton on 18/9 with yellow bill, 4 bold bars on remiges and extensive dark on wing-tips.
Newsome (2009). Photographs are shown of 2 birds, neither aged in the caption, which appear to be a pale-phase juvenile at Sunderland on 14/9 and a dark-phase juvenile at Whitburn on 17/9.
In general juveniles are more likely to be photographed than adults as their weaker flight causes them to fly at lower altitude. So the presence of 2 adult females in a sample of 7 does suggest that the movement was by no means purely of juveniles. On the UK side it is not thought that juveniles alone would migrate from East Anglia to the coast of Benelux, but in mixed-age flocks juveniles may have accompanied to some extent an adult-led move. Only adults could have known that land would be found 160-180 km to the east. Juveniles alone would have continued southwards into south east England until forced to cross the sea, as in 2000. So the proportion of juveniles in SE England is higher than in NE England, because adults turned E across the North Sea from East Anglia. An explanation for the number of adults present is the very late breeding season in northern Britain where sites held 2 adults and 2 fledged juveniles only a few days before the start of the movement. Adults moving en masse at this late stage in the season is remarkable but their exit had been blocked by poor weather in the few days before.
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.