Data from: Around the Mediterranean: an extreme example of loop migration in a long-distance migratory passerine

An important issue in migration research is how small-bodied passerines pass over vast geographical barriers; in European-African avian migration, these are represented by the Mediterranean Sea and the Sahara Desert. Eastern (passing Eastern Mediterranean), central (passing Apennine Peninsula) and western (via western Mediterranean) major migration flyways are distinguished for European migratory birds. The autumn and spring migration routes may differ (loop migration) and there could be a certain level of individual flexibility in how individuals navigate themselves during a single migration cycle. We used light-level loggers to map migration routes of barn swallows Hirundo rustica breeding in the centre of a wide putative contact zone between the north-eastern and southern-western European populations that differ in migration flyways utilised and wintering grounds. Our data documented high variation in migration patterns and wintering sites of tracked birds (n = 19 individuals) from a single breeding colony, with evidence for loop migration in all but one of the tracked swallows. In general, two migratory strategies were distinguished. In the first, birds wintering in a belt stretching from south-central to southern Africa that used an eastern route for both the spring and autumn migration, then shifted their spring migration eastwards (anti-clockwise loops, n = 12). In the second, birds used an eastern or central route to their wintering grounds in central Africa, shifting the spring migration route westward (clockwise loops, n = 7). In addition, we observed an extremely wide clockwise loop migration encompassing the entire Mediterranean, with one individual utilising both the eastern (autumn) and western (spring) migratory flyway during a single annual migration cycle. Further investigation is needed to ascertain whether clockwise migratory loops encircling the entire Mediterranean also occur other small long-distance passerine species.

迁徙研究的核心议题之一，是小型雀形目鸟类如何跨越广袤的地理屏障；在欧洲-非洲鸟类迁徙体系中，这类屏障具体表现为地中海与撒哈拉沙漠。欧洲候鸟可划分为三条主要迁徙通道：东线（途经东地中海）、中线（途经亚平宁半岛）与西线（经西地中海）。春秋两季的迁徙路线或存在差异，这类现象被称为环形迁徙（loop migration），且个体在单次迁徙周期内的导航策略可具备一定程度的灵活性。本研究借助光照记录仪（light-level loggers），对繁殖于东北欧与西南欧种群的广阔潜在接触带中心区域的家燕（Hirundo rustica）进行了迁徙路线测绘——上述两个种群在迁徙通道选择与越冬地偏好上均存在差异。研究数据显示，来自同一繁殖群的19只被追踪个体，其迁徙模式与越冬地存在显著差异；除1只个体外，其余所有被追踪家燕均呈现出环形迁徙特征。总体而言，可区分出两种迁徙策略：第一种策略下，家燕的越冬地为非洲中南部至南部非洲的狭长地带，春秋两季均采用东线迁徙，且春季迁徙路线向东偏移，即逆时针环形迁徙，共涉及12只个体。第二种策略下，家燕通过东线或中线抵达非洲中部的越冬地，春季迁徙路线则向西偏移，即顺时针环形迁徙，共涉及7只个体。此外，本研究还观测到一条覆盖整个地中海的超大顺时针环形迁徙路线，其中1只个体在单次年度迁徙周期内，同时使用了东线（秋季）与西线（春季）迁徙通道。未来仍需开展进一步研究，以确认环绕整个地中海的顺时针环形迁徙模式是否也存在于其他小型长距离雀形目鸟类中。