One million raptors: But where do they come from?
A flock of Black Kites soaring in front our watch sites. Nowadays we count around 300.000 individuals each autumn! Photo by Marc Heetkamp
As a migration counter, you can’t help but wonder: “Where did the bird I just tallied come from, and where is it going?” In light of BRC’s main aim — monitoring raptor population trends — this very question is crucial, because we want to know which populations we are actually monitoring.
GPS loggers that track a bird’s every movement may seem like the ultimate solution. Indeed, even in some of our earlier work (Zaytseva et al. 2021), we made a first attempt to estimate the origins of breeding populations based on the limited tracking studies available for the East African–Eurasian flyway.
Black Kite photographed at Batumi in September 2018, a bird that had been wing- and GPS-tagged at the Dudaim landfill, Israel. Photo by Wim Bovens.
A recently published study by Efrat et al. (2025) takes this a step further. It represents the most comprehensive effort to date, leveraging large GPS-tracking datasets to validate the monitoring potential of migration counts at two well-established watch-sites in the East African-Eurasian flyway: Eilat, Israel, and Batumi, Georgia. For Batumi, for example, the study shows that our watch-sites are well-positioned to monitor a large proportion of Black Kites from a major non-breeding population in the Negev Desert that breeds over a wide longitudinal range stretching over Ukraine and western Russia.
At BRC, we greatly appreciate the efforts of Efrat et al. (2025) to assess the monitoring potential of our surveys using an independent tracking dataset, and read their work with great interest. After some internal discussion in the team, we concluded that Efrat et al. (2025) offer an invaluable first step towards better understanding patterns of migratory connectivity flyway. At the same time, we felt the bias towards very specific populations of tracked birds comes with important caveats regarding the interpretation of watch-site monitoring potential, and that the authors left several important opportunities for integrating count and tracking efforts underexplored. After an informal discussion with lead-author Ron Efrat, we have now published a commentary letter further elaborating on these gaps and opportunities:
(If you do not have access to the subscription article, you can find the accepted manuscript version of the commentary here)
Efrat et al. (2025), in turn, were offered the opportunity to respond to our commentary, and you can find their reflections here.
Overall, we found it a very interesting experience to publish our scientific debate in such a public manner, via a commentary piece, and we thank Efrat and his co-authors for a constructive and cordial discussion. Even if some minor disagreements remain, it is abundantly clear that large tracking datasets can and should be used to clarify the population-level origins and destinations of birds counted at key watch-sites.
Moreover, besides the question of how robust migration counts are as a monitoring tool, we emphasise that the utility of migration count projects such as BRC takes a multifaceted form, including capacity-building, environmental education, awareness raising and other aspects that contribute to raptor conservation in a broader sense. Even in the “golden age” of animal tracking, old-school migration counts have an important role to play in connecting people with raptors — perhaps now more than ever.
