Accounting for differential migration strategies between age groups to monitor raptor population dynamics in the eastern Black Sea flyway

Migration counts can offer a cost-effective method for monitoring the state of migrant raptor populations. However, differential migration strategies between inexperienced juveniles and experienced non-juveniles are rarely accounted for when inferring population trends from raptor migration counts. Since 2011, the Batumi Raptor Count (BRC) monitors the autumn migration of more than 1 million raptors along the eastern Black Sea coast in the Republic of Georgia. We also systematically sampled age information to assess differential migration timing between age groups and estimate age-specific linear trends in abundance between 2011 and 2018 for eight focal species. In so doing we aimed (1) to reassess the global relevance of BRC counts for each species and the potential for monitoring abundance of juveniles and non-juveniles, and (2) to identify demographic changes underlying recent trends in overall abundance. We found that the mean annual passage of non-juveniles at Batumi represents at least 1% of the estimated global breeding population of five study species. As expected, counts of juveniles were more variable than counts of non-juveniles. Yet despite our short monitoring period our models had sufficient statistical power to detect changes in abundance of 10%/year or less for at least one age group in all species except Pallid Harrier Circus macrourus. Our results indicate stable abundance and demography for half of the study species. We also found strong and significant increases in the abundance of Black Kites Milvis migrans and Short-toed Eagles Circaetus gallicus that were primarily due to increasing numbers of non-juveniles. By contrast, juvenile Montagu’s Harriers Circus pygargus and Booted Eagles Hieraaetus pennatus significantly decreased in abundance. The first decade of BRC surveys offers an important benchmark for monitoring raptor populations using the eastern African– Palearctic flyway in the 21st century. We discuss possible causes of the observed trends and hope our work will stimulate demographic monitoring at migration count sitesPeer reviewe