European Honey Buzzard

September 2, 2017

“The gates of the Caucasus have opened and the Honey Buzzards are upon us! The show started around 11:30 when, after our visibility cleared after a rain shower, the north was filled with birds. Most of the migrants chose to follow the coastline with more than 9 000 birds passing within just 20 minutes!“

Average count per season: 500 066
Highest season count: 656 171 in 2014
Record day: 178 796 on September 3, 2012 

European Honey Buzzard (hereafter Honey Buzzard) is a raptor species breeding in the forests from Central Asia to Western Europe and spending the non-breeding season over a wide-ranging area in sub-Saharan Africa.

Batumi, situated on the east African-Palearctic flyway, is the bottleneck where the majority of Honey Buzzards in the world is funnelled through on their annual movement from breeding to non-breeding grounds [1]. Interestingly, we count roughly 20% more adult birds on autumn migration than there are estimated to be breeding in the entire global range of this species [2]. This striking result underlines the importance of our monitoring effort in providing more accurate population size estimates for this notoriously secretive woodland bird, and especially in the understudied areas where the birds passing Batumi originate from.

After 12 years of raptor counts, we can say for certain that the passage time of Honey Buzzards in Batumi is very predictable: 90% of the adult birds pass during the last week of August and the first week of September (with only slight variability in peak dates caused by weather conditions). By contrast, and consistent with observations in the Mediterranean region [3], the bulk of the juveniles migrate independently from adults around ten days later (Figure 1) [1]. Over these years, we have witnessed no trends in the timing of migration of adults, which points to a lack of response to global change.

Such a clear age-specific migration timing is unique among the raptor species migrating through the Batumi bottleneck. Furthermore, juveniles do not only differ in timing, but also pass in very different numbers. Whereas adults clearly avoid large water bodies and high mountains, making them form huge aggregations in Batumi, we record only a low percentage of juveniles (ca 5,9% of all individuals). It has been shown by satellite-tracking that the inexperienced first year birds, following their innate direction, disperse more [4] and take more direct routes, crossing seas and other inhospitable environments that adults have learned to avoid [5]. There is a considerable passage of adult Honey Buzzards across the Central and eastern Mediterranean, but these birds still make shorter sea-crossings compared to juveniles [3,6]. After their first southward migration, most of the juvenile birds spend at least 18 months in Africa, before returning to Europe for the first time [7]. As a result, in Batumi we only observe birds in juvenile or adult plumage during autumn. However, on the 17th of September 2017, John Wright observed and photographed a 2nd calendar year Honey Buzzard, which as far as we know, is the first ever documented record of an immature bird during autumn in the Western Palearctic [8].

While the IUCN Red List estimates the population size of Honey Buzzard to be decreasing [9], we do not see any trends in the number of Honey Buzzards migrating through Batumi, indicating a stable source population. Unfortunately, the number of juvenile birds passing through the bottleneck varies greatly between years, and the migration behaviour of juveniles makes it difficult to make any inferences about their breeding success. Nevertheless, we are confident counts of adult birds offer a good indicator of the wellbeing of this long-lived species in the East African-Palearctic flyway [1].

By now we have a good overview of the autumn migration of Honey Buzzards, but we are just starting to gain more knowledge on the spring passage of this species in Batumi. The spring counts performed in 2019 and 2020 differed greatly, with 89 000 and 44 000 birds tallied, respectively. The relatively low counts for the species in spring indicates that Honey Buzzards use different migration corridors during autumn and spring journeys. One possibility is that seasonal conditions allow Honey Buzzards to cross the Caucasus 100 km or further inland in spring vs autumn. Another possibility is that adults use completely different flyways in spring. Such behaviour has been found in GPS-tracked Honey Buzzards breeding in Germany [10] and Hungary [11] that used the Straits of Gibraltar in autumn while returning across the Mediterranean or via the Italian peninsula in spring. Such flyway-switching also appears to occur in adult Honey Buzzards breeding in Finland and migrating via Batumi. However, interpreting the large seasonal difference in Honey Buzzard counts at Batumi, as well as trends in migration counts, will require more concerted efforts to map seasonal connectivity between migration watch-sites and breeding areas throughout the species’ eastern range. 

The autumn passage of Honey Buzzard in Batumi, with wide streams of these birds lasting for hours on end, is an experience you will always remember!   

References

1. Vansteelant, W. M. G., Wehrmann, J., Engelen, D., Jansen, J., Verhelst, B., Benjumea, R., … Hoekstra, B. (2020). Accounting for differential migration strategies between age groups to monitor raptor population dynamics in the eastern Black Sea flyway. Ibis, (162), 356–372.

2. BirdLife International. (2020). Species factsheet: Pernis apivorus. Downloaded from http://www.birdlife.org on 09/08/2020.

3. Agostini, N., C. Caleiro., & Panuccio, M. (2004). Analysis of the autumn migration of juvenile Honey-buzzard (Pernis apivorus) across the Central Mediterranean. Journal of Raptor Research, 38(3), 283-286.

4. Vansteelant, W. M. G., Kekkonen, J., & Byholm, P. (2017). Wind conditions and geography shape the first outbound migration of juvenile honey buzzards and their distribution across sub-Saharan Africa. Proceedings of the Royal Society B.

5. Hake, M., Kjelle, N., & Alerstam, T. (2003). Age-dependent migration strategy in honey buzzards Pernis api 7 orus tracked by satellite. Oikos, 103(2), 385–396.

6. Nourani, E., Vansteelant, W., Byholm, P., & Safi, K. (2020). Dynamics of the energy seascape can explain intra-specific variations in sea-crossing behaviour of soaring birds. Biology Letters, 16(1).

7. Corso, A., & Panuccio, M. (2012). The status of second-calendar-year Honey-Buzzards in Europe. British Birds, 105(8), 484-486.

8. Wright, J., Cavaillès, S., & Vansteelant, W. M. G. (2019). Photographic evidence of a 2nd calendar- year female European Honey Buzzard Pernis apivorus on autumn migration in the Western Palearctic. Sandgrouse, 41, 101-104.

9. BirdLife International. (2016). Pernis apivorus. The IUCN Red List of Threatened Species 2016: e.T22694989A93482980. https://dx.doi.org/10.2305/IUCN.UK.2016-3.RLTS.T22694989A93482980.en. Downloaded on 09 August 2020.

10. Meyburg et al. (2010). On the biology of the Honey Buzzard (Pernis apivorus)–results of satellite tracking. Poster at the 7th international symposium “Population Ecology of Raptors and Owls”, Halberstadt, Germany.

11. Agostini, N., Prommer, M., & Panuccio, M. (2019). Repeated large scale loop migrations of an adult European Honey Buzzard. Avocetta, 43(1), 13-21.