Across the deserts and sea: inter-individual variation in migration routes of south-central European barn swallows (Hirundo rustica)

Abstract

Background The spatiotemporal organization of migratory routes of long-distance migrants results from trade-offs between minimizing the journey length and en route risk of migration-related mortality, which may be reduced by avoiding crossing inhospitable ecological barriers. Despite flourishing avian migration research in recent decades, little is still known about inter-individual variability in migratory routes, as well as the carry-over effects of spatial and temporal features of migration on subsequent migration stages. Methods We reconstructed post- and pre-breeding migration routes, barrier crossing behaviour and non-breeding movements of the largest sample (N = 85) analysed to date of individual barn swallows breeding in south-central Europe, which were tracked using light-level geolocators. Results Most birds spent their non-breeding period in the Congo basin in a single stationary area, but a small fraction of itinerant individuals reaching South Africa was also observed. Birds generally followed a ‘clockwise loop migration pattern’, moving through the central Mediterranean and the Sahara Desert during post-breeding (north to south) migration yet switching to a more western route, along the Atlantic coast of Africa, Iberia and western Mediterranean during the pre-breeding (south to north) migration. Southward migration was straighter and less variable, while northward migration was significantly faster despite the broader detour along the Atlantic coast and Iberia. These patterns showed limited sex-related variability. The timing of different circannual events was tightly linked with previous migration stages, considerably affecting migration route and speed of subsequent movements. Indeed, individuals departing late from Africa performed straighter and faster pre-breeding migrations, partly compensating for the initial departure delays, but likely at the cost of performing riskier movements across ecological barriers. Conclusions Different spatiotemporal migration strategies during post- and pre-breeding migration suggest that conditions en route may differ seasonally and allow for more efficient travelling along different migration corridors in either season. While highlighting patterns of inter-individual variability, our results support increasing evidence for widespread loop migration patterns among Afro-Palearctic avian migrants. Also, they suggest that carry-over effects acting across different phases of the annual cycle of migratory species can have major impacts on evolutionary processes.

Early start, early finish – Migration strategy of tropical Barn Swallows (Hirundo rustica)

“…Crossing ecological barriers is one of the most energy-demanding and risky stages in bird migration [43]. Previous studies have found that Barn Swallow populations in Italy and Switzerland typically follow a "clockwise loop migration" pattern, crossing barriers through the central Mediterranean and the Sahara Desert during autumn migration, but shifting to a more western, longer route during spring migration, bypassing the barriers [44]. This study found that the migratory pattern of the breeding population of Barn Swallows in Zhanjiang exhibited a contrary "counter-clockwise loop migration" pattern, with the majority of individuals choosing a western route bypassing the South China Sea to reach the wintering grounds via Vietnam during autumn migration, while during spring migration, they directly crossed the South China Sea, taking a shorter route.…”

Local Variability of Trace Element Concentration in Barn Swallow (Hirundo rustica) Nestlings from the Po Plain (Northern Italy)

“…The barn swallow is a small migratory passerine bird whose breeding range encompasses the whole Holarctic region. Italian populations spend the non-breeding period mostly in west-African sub-Saharan countries [44,45]. This species breeds synanthropically in rural buildings and forages on aerial insects captured within 500 m of the breeding sites [43].…”

The effects of cities on quail (Coturnix coturnix) migration: a disturbing story of population connectivity, health, and ecography

“…The locations, size, connectivity, and activities of mega-urbanized areas have spread to influence every ecosystem, to the point that One Health for One Planet initiatives require urgent implementation (Barroso et al 2023 ; Fastré et al 2021 ; Yin et al 2022 ). For example, migratory birds must cross these altered biomes to complete their biological cycles and their survival depends on their relationships with ecosystems under pressure (Pancerasa et al 2022 ). Indeed, ecosystems, agrosystems, and mega-urbanizations are now so entangled that the significant flow of materials and organisms between them creates widespread disturbances, such as changes in pollutants, diseases, or predation rates (Krauze and Wagner 2019 ).…”