Full-season spring migration counts reveal seasonal contrasts in raptor migration through the eastern Black Sea flyway

Abstract

The extent to which geographical features like coastlines and mountain ranges funnel migrating birds depends on the seasonal context and direction of migration. Near Batumi, in the Republic of Georgia, the eastern Black Sea coast and Lesser Caucasus funnel over one million raptors through a 10-20 km wide coastal strip every autumn. The funnelling effect of the Lesser Caucasus appears much less evident for northbound migrants. Yet historical data suggest tens of thousands of raptors pass through the region in spring. To elucidate the composition and timing of spring raptor migration we conducted full-season migration surveys near Batumi in 2019, 2020 and 2022. In total, we recorded 33 species and, on average, counted 542,161 raptors (min. 455,799 - max. 618,848) annually. The bulk of the spring passage consisted of Black Kite Milvus migrans (239,649 +- 22,547) and Steppe Buzzard Buteo buteo vulpinus (194,029 +- 102,702). The most diverse and intense spring migration occurred from late March through mid-April, when the median passage date of 12 of 15 common species (>100 ind. y-1) occurred. Species with longer autumn migration periods tended to have longer spring migration periods, and most species had a longer migration period in spring than in autumn, likely due to larger age differences in timing during spring. Species abundance was up to an order of magnitude lower in spring than in autumn, consistent with a weaker bottleneck-effect in spring. Nevertheless, our results confirm the eastern Black Sea coast as a principal spring flyway and help redraw the map of East African-Eurasian migration for several Palearctic raptors. While we will not continue annual spring migration surveys, our data provides a baseline for detecting changes in raptor migration through short-term surveys and can help plan migration-based conservation and research at Batumi in spring.