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

Abstract

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 sites.

Intratropical migration of the Hook-billed Kite ( Chondrohierax uncinatus ) in Middle America

“…The presence of some Hook-billed Kites year-round in the Rio Grande Valley of Texas suggests that at least the population at the northern end of its range is a combination of facultative migrants and resident individuals (Brush 2005, Clark and Schmitt 2017, Greeney and Boesman 2022). Further, most obligate migratory raptor species in both tropical and boreal regions exhibit a differential migration where the timing of age and sex classes differ (Kjellén 1992, Goodrich and Smith 2008, Hull et al 2012, Vansteelant et al 2020. In Belize, we found no difference in the timing between age and sex classes, which supports the suggestion that the northern populations are facultative migrants.…”

Raptor migration at Guantouling, south-west China: phenology, weather influence and persecution pressure

“…We calculated the three-day average, date of the first 5%, 50% and 95% passage, and main migration period (central 90% of migration, first 5% – first 95%) from 5 October to 6 November (each date with the observations aggregated from five years) for the 10 most abundant species, excluding locally common species (Vansteelant et al 2020). Locally common species were not included in this analysis because occurrence of resident individuals can interfere with the phenology estimates.…”

“…This results in migration bottlenecks that attract a large number of raptors, providing opportunities for long-term studies into population trend, timing of migration, influence of weather, and other aspects of raptor ecology that are otherwise difficult to survey (Bednarz et al 1990, Farmer et al . 2007, Panuccio et al 2017, Vansteelant et al 2020). However, such behaviour also leads to mass and often illegal trapping and killing of raptors at the migration bottlenecks (van Maanen et al 2001, Bildstein 2006, Dalvi et al 2013, Brochet et al 2016).…”

“…Therefore, migrating raptors often concentrate along geographical corridors with favourable thermal conditions to minimise energy cost (Mueller andBerger 1967, Bildstein 2006), with their timing and number often shaped by the local weather conditions (Richardson 1990). This results in migration bottlenecks that attract a large number of raptors, providing opportunities for long-term studies into population trend, timing of migration, influence of weather, and other aspects of raptor ecology that are otherwise difficult to survey (Bednarz et al 1990, Farmer et al 2007, Panuccio et al 2017, Vansteelant et al 2020. However, such behaviour also leads to mass and often illegal trapping and killing of raptors at the migration bottlenecks (van Maanen et al 2001, Bildstein 2006, Dalvi et al 2013, Brochet et al 2016.…”