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（small-bodied passerines）如何跨越广袤的地理屏障（geographical barriers）；在欧洲-非洲鸟类迁徙系统中，这类屏障以地中海（Mediterranean Sea）与撒哈拉沙漠（Sahara Desert）为典型代表。欧洲迁徙鸟类可划分为三大主要迁徙通道（migration flyways）：东部通道（途经东地中海）、中部通道（途经亚平宁半岛）与西部通道（经地中海西部）。春秋两季的迁徙路线可能存在差异（即环形迁徙（loop migration）），且在单个迁徙周期内，个体在导航过程中具备一定程度的灵活性。本研究利用光感记录仪（light-level loggers），对繁殖于东北欧与西南欧种群（二者在所利用的迁徙通道及越冬地（wintering grounds）均存在差异）的广阔推测接触区（putative contact zone）中心的家燕（Hirundo rustica）的迁徙路线进行了测绘。我们的数据记录了来自同一繁殖种群（breeding colony）的19只追踪个体的迁徙模式与越冬地的高度变异，且除1只家燕外，其余个体均存在环形迁徙现象。总体而言，可区分出两类迁徙策略（migratory strategies）：第一类，越冬于中南至南非地带的个体，春秋两季均使用东部迁徙通道，春季迁徙时会向东偏移（逆时针环形迁徙（anti-clockwise loops），n=12）；第二类，个体利用东部或中部通道前往中非越冬地，春季迁徙时则向西偏移（顺时针环形迁徙（clockwise loops），n=7）。此外，我们观察到一条覆盖整个地中海的极端宽幅顺时针环形迁徙路线，1只个体在单个年度迁徙周期（annual migration cycle）内同时利用了东部（秋季）与西部（春季）迁徙通道。未来仍需开展进一步研究，以明确环绕整个地中海的顺时针环形迁徙是否同样存在于其他小型长距离雀形目鸟类中。