comparing migration strategies of marsh harriers from the Low Countries and southern sweden

The adult male Marsh Harrier ‘Walter’ was equipped with an UvA-BiTS GPS logger at his breeding site in Flandres (Belgium). - Photo by Anny Anselin (INBO) — **The adult male Marsh Harrier ‘Walter’** was equipped with an UvA-BiTS GPS logger at his breeding site in Flandres (Belgium). - Photo by Anny Anselin (INBO)

While the number of tracking studies on migrant raptors is rapidly increasing new opportunities are emerging to make comparative studies of migratory behaviour across multiple populations of certain species. I conducted this projects as a free-lance ecologist in collaboration with researchers of the Knowledge Center Grauwe Kiekendief (Netherlands), the Research Institute for Nature and Forest (Flandres, Belgium) and Lund University (Sweden) to make such a comparative analysis of migration corridors and schedules across breeding populations of Marsh Harrier (Circus aeruginosus) in the Low Countries (Flandres and The Netherlands) and Southern Sweden.

Our analyses revealed a marked separation of autumn migration corridors of LC and Swedish harriers all throughout the West African-Eurasian flyway. In spring, harriers departed through overlapping migration corridors, but according to very different schedules depending on their breeding population. More specifically, harriers breeding in the Low Countries departed much earlier from West Africa, and made long stop-overs in NW Africa, whereas Swedish birds depart later and forego spring stop-overs, thereby catching up with the LC harriers in NW Africa, and finally arriving at their respective breeding areas around a similar time. These are remarkable differences considering the proximity of their breeding areas, and them converging at particular hot-spots in West Africa (e.g. Inner Niger Delta, Senegal River, coastal wetlands).

In a more general sense, our study highlights how difficult it is to generalise known migration strategies from one breeding population to another (Standberg et al 2008, Klaassen et al 2010). Our article was published open access in Journal of Ornithology (Vansteelant et al. 2020) along with accompanying data and code.

West African non-breeding sites of adult Marsh Harriers that were GPS-tracked from breeding areas in Belgium (BE), the Netherlands (NL) and Sweden (SW). Birds from all three areas have a largely overlapping non-breeding range. However, our analyses … — **West African non-breeding sites of adult Marsh Harriers** that were GPS-tracked from breeding areas in Belgium (BE), the Netherlands (NL) and Sweden (SW). Birds from all three areas have a largely overlapping non-breeding range. However, our analyses (in prep) reveal surprising differences in migration corridors and schedules between Marsh Harriers breeding in the Low Countries (BE, NL) vs. those breeding in Sweden (SW).

batumi raptor count: CONSOLIDATION OF A CITIZEN SCIENCE PROJECT FOR LONG-TERM MONITORING OF RAPTOR MIGRATION

Peak passage of European Honey Buzzards at the BRC count station in Sakhalvasho (Georgia) on 2 Sept 2017. - video by Elien Hoekstra

Hundreds of thousands of raptors converge through geographical bottlenecks such as sea-straits on their seasonal migrations between Europe and Africa. For some species the passage at these sites represents a large fraction of regional populations, so that systematic migration counts can be a useful tool for monitoring. My research career started as co-founder of Batumi Raptor Count (BRC), where we developed a standardised protocol for monitoring the autumn passage of more than one million raptors along the eastern Black Sea coast of the Republic of Georgia. As research coordinator I lead the effort to analyse over a decade of monitoring data and played a key role in funding, conceptualising and implementing data recording, processing and analyses.

After publishing our initial assessment of the global relevance of the Batumi passage for various species (Verhelst et al. 2011), we went on to fully digitise our data recording and to automate data management. Following the 10th anniversary of the BRC project in 2017, and with the help of the Dutch-Georgian Ornithological Foundation (DuGOF), we secured an NLBIF grant to fund the open access publication of the BRC dataset through the Global Biodiversity Information Facility (GBIF). In the accompanying Zookeys data paper we detail our automated data recording and processing protocol, and describe important pitfalls and best practices for end users (Wehrmann et al. 2019).

The NLBIF grant also funded a first analysis of changes in abundance, demography and timing for 8 target species in the BRC monitoring, published in Ibis (Vansteelant et al. 2020). With this paper consolidated BRC´s scientific approach to estimating age-specific trends from migration count data, and established a solid benchmark for the volunteer-based monitoring of raptor migration in the Batumi bottleneck during the 21st century.