The Western marsh harrier Circus aeruginosus is a partial migrant with populations breeding in eastern and northern Europe migrating south and wintering mostly in sub-Saharan Africa. During autumn migration harriers move on a broad front heading SW and undertaking long sea-crossings en route to their wintering quarters. However, a substantial proportion of birds, mostly juveniles and adult females, migrate only shorter distances wintering in Europe and North Africa. In a 7-year study (2011–2017), between 26 August and 30 September, we compared the ratio and timing of the different sex/age classes of Western marsh harriers (adult male, adult female, juvenile) at two migration bottlenecks: the Strait of Messina (southern Italy) and Batumi (Georgia). At both sites, adult males migrated slightly earlier that adult females and earlier than juveniles. Among adults, males outnumbered females and this difference was more evident along the eastern flyway. At Batumi, a higher proportion of juveniles (first calendar year birds) was reported. The male-biased sex ratio reported at the Strait of Messina is very similar to that reported in broods of Western marsh harriers breeding in The Netherland and Poland. We suggest that the higher proportion of adult males recorded at Batumi could reflect the sex ratio in male-biased populations in western Russia.

中文翻译：

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西部沼泽鹞性别和年龄组秋季迁徙差异：纵向模式分析

西部沼泽鹞 Circus aeruginosus 是一种部分迁徙动物，在东欧和北欧繁殖，主要向南迁徙并在撒哈拉以南非洲越冬。在秋季迁徙期间，鹞鹞在宽阔的前沿向西南方向移动，并在前往越冬区的途中进行长途跨海。然而，相当大比例的鸟类，主要是幼鸟和成年雌鸟，在欧洲和北非越冬的距离较短。在 8 月 26 日至 9 月 30 日之间的一项为期 7 年的研究（2011-2017 年）中，我们比较了西部沼泽鹞（成年雄性、成年雌性、幼年）在两个迁徙瓶颈处的不同性别/年龄等级的比例和时间：墨西拿海峡（意大利南部）和巴统海峡（格鲁吉亚）。在这两个地点，成年雄性比成年雌性稍早迁移，也早于幼体。在成年人中，雄性多于雌性，这种差异在东部飞行路线上更为明显。在巴统，据报道有更高比例的幼鸟（第一年的鸟类）。在墨西拿海峡报道的雄性性别比例与在荷兰和波兰繁殖的西方沼泽鹞子群中报道的非常相似。我们认为巴统记录的成年男性比例较高可能反映了俄罗斯西部男性偏向人群的性别比例。