Black Kites streaming past the Sakhalvasho count station

Raptor Migration

Trends

Millions of raptors, thousands of hours, hundreds of volunteers from dozens of countries: this is what we learned from 8 years of BRC autumn counts.

Trends for 8 important species: Honey Buzzard, Montagu's Harrier, Pallid Harrier, Marsh Harrier, Booted Eagle, Lesser Spotted Eagle, Short-toed Eagle, Black Kite.

For 8 important species for the bottleneck, we have analysed trends over the past 8 years of standardised counts (2011-2018). Despite this short study period, we can already detect moderate changes in abundance for at least one age class in all species except Pallid Harrier. Here we summarise these results, but you can find out more in our paper published in Ibis.

Read the paper in Ibis

Vansteelant, W. M., Wehrmann, J., Engelen, D., Jansen, J., Verhelst, B., Benjumea, R., Cavaillès, S., Kaasiku, T., Hoekstra, B. and de Boer, F. (2019), Accounting for differential migration strategies between age groups to monitor raptor population dynamics in the eastern Black Sea flyway. Ibis. doi: 10.1111/ibi.12773

A kettle of Honey Buzzards migrating over the Batumi bottleneck

Age data is barely used in population trend analyses, but…

Inexperienced juveniles often behave differently than more experienced non-juveniles. Demographic changes may serve as an early warning system for population change.

To really understand our study populations, we have to distinguish between age classes (juveniles and non-juveniles).

Juveniles Non-juveniles Colour-coded throughout, matching the charts below.

Montagu's Harrier

Circus pygargus

Proportion of world population via Batumi: 0,9 - 4,4%
Phenology: No change detected in phenology

Graph of Montagu's Harrier counts by age class at Batumi, 2011-2018 — Too high variability to detect moderate (< 10%) trends in 8 years of counts.
Strong decline in juveniles of 17% per year.

Pallid Harrier

Circus macrourus

Proportion of world population via Batumi: 2,5 - 4,1%
Phenology: No change detected in phenology

Graph of Pallid Harrier counts by age class at Batumi, 2011-2018 — Inconsistent proportion of source population passes Batumi in autumn.
Variability too high to detect trends.
Counting Pallids remains necessary to estimate proportions of Montagu's in unidentified harriers.

Marsh Harrier

Circus aeruginosus

Proportion of world population via Batumi: 0,4 - 0,8%
Phenology: No change detected in phenology

Graph of Marsh Harrier counts by age class at Batumi, 2011-2018 — No apparent trends, indicative of a stable source population.
Longest migration period of all study species.

Honey Buzzard

Pernis apivorus

Proportion of world population via Batumi: 119,1 - 178,6%
Phenology: No change detected in phenology

Graph of Honey Buzzard counts by age class at Batumi, 2011-2018 — No apparent trend for non-juveniles, indicative of a stable population.
Too high variability of juveniles to detect moderate trends.

Booted Eagle

Hieraaetus pennatus

Proportion of world population via Batumi: 2,7 - 3,4%
Phenology: No change detected in phenology

Graph of Booted Eagle counts by age class at Batumi, 2011-2018 — Obscured by a seemingly stable population, is a strong decline in the number of juveniles (-10% per year).
Until 2018 there was even a significant increase in the number of non-juveniles (P < 0.05).

Short-toed Eagle

Circaetus gallicus

Proportion of world population via Batumi: 0,6 - 1,2%
Phenology: No change detected in phenology

Graph of Short-toed Eagle counts by age class at Batumi, 2011-2018 — Very small variability in the numbers of non-juveniles, shows a slightly increasing trend (+3% per year).
Too high variability in the number of juveniles to detect any trends.

Lesser Spotted Eagle

Clanga pomarina

Proportion of world population via Batumi: 10,3 - 15,5%
Phenology: No change detected in phenology

Graph of Lesser Spotted Eagle counts by age class at Batumi, 2011-2018 — No apparent trends, indicative of a stable population.

Black Kite

Milvus migrans

Proportion of world population via Batumi: 3,8 - 9,6%
Phenology: No change detected in phenology

Graph of Black Kite counts by age class at Batumi, 2011-2018 — Clearly increasing population (+10% per year), mostly driven by an increase in non-juveniles (+11% per year).
Increase in juveniles only just significant since 2018 (P = 0.05, +9% per year).

Thousands of Black Kites wintering at the Dudaim landfill in the Negev desert, Israel

Why do the numbers of Black Kites increase so much?

Since 2000, there has been a strong increase from 1,000s to 30,000+ Black Kites wintering on Dudaim landfill in the Negev desert, Israel — and there are similar situations in SE-Europe and Turkey.

Photo: Video: Richard Stein

Batumi – Negev

There is a strong connection between Batumi and the Negev desert, as shown by tracking studies (Daniel Berkowic, unpublished) and a sighting of a wingtagged Black Kite on the Sakhalvasho station.

GPS track map showing a tagged Black Kite moving between the Negev desert and the Batumi bottleneck — GPS track of one of Daniel Berkowic's Black Kites, clearly showing that birds wintering in the Negev could end up moving through the Batumi bottleneck on spring and autumn migration.

Steppe Buzzards streaming through low cloud over the Batumi bottleneck

Conclusion

Most of the studied raptor populations are stable. European Honey Buzzard, Western Marsh Harrier, Booted Eagle, Lesser Spotted Eagle.
Clear increase in the numbers of Black Kite and Short-toed Eagle. Mostly as a result of increasing numbers of non-juvenile birds, which is indicative of improved wintering conditions.
Stable totals of Montagu's Harrier and Booted Eagle obscure the significant declines in the numbers of juvenile birds. A decline in the numbers of juveniles is indicative of a problem at the breeding grounds.

Interested in all the details?

Read the entire paper in Ibis

BRC counters and volunteers gathered at a count station

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