The phenomenon of over‐summering in southern non‐breeding areas by boreal‐breeding birds is particularly prevalent among shorebirds. Despite its frequency, it is understudied compared with most other aspects of shorebird ecology. Our aim was to expand knowledge of this subject through a study of red knots Calidris canutus rufa over‐summering at a site in Argentina during the austral winter. We measured the proportion of one‐year‐old and adult over‐summerers and evaluated the roles of flight‐feather condition and physiological status (through leukocite profile, physiological stress index and presence of blood parasites) as triggers for over‐summering. We also explore sex‐ratio, the evolution of body mass and extent of breeding plumage within each age‐class. Over‐summerers were 57% yearlings and 43% adults, 46% females and 54% males. Almost all yearlings exhibited incomplete molt of primary feathers and some were in active primary molt. This suggests that the condition of flight‐feathers and the timing of molt are likely to be important factors selecting for deferred migration during the first year of life. Other factors, not associated with flight‐feather molt, seem to trigger over‐summering in adults, which had completed flight feather molt but had low fat loads and/or incomplete alternate plumage. We found no evidence of a weakened immune system, high loads of blood parasites or high stress levels that can explain this poor migratory conditioning and therefore over‐summering. Our data indicates that our adult age‐class comprises both young individuals postponing first breeding until they are at least two years old and sexually mature individuals with prior reproductive experience skipping a breeding opportunity. Breeding propensity and age at first breeding are both poorly known, but key demographic parameters that determine population growth. This study suggests that potentially they can be estimated from mark–recapture at non‐breeding areas and this warrants further study.

中文翻译：

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是否迁移：长距离迁徙水鸟过度繁殖的驱动力

北方非繁殖区过度繁殖的鸟类过度繁殖的现象在shore鸟中尤为普遍。尽管它的频率很高，但与水鸟生态学的大多数其他方面相比，它的研究仍很少。我们的目标是通过对红色结节Calidris canutus rufa的研究来扩展该主题的知识。在南方冬季，在阿根廷的某处过分盛行。我们测量了一岁和成年超龄者的比例，并评估了飞羽状况和生理状态（通过白血球概况，生理压力指数和血液中寄生虫的存在）的作用，这些因素是造成超龄现象的诱因。我们还探讨了每个年龄段的性别比例，体重的演变以及繁殖羽毛的范围。一岁以上的人是一岁的57％，成年的43％，女性46％，男性54％。几乎所有的一岁鸽都表现出不完全的原生羽毛蜕皮，而有些则处于活跃的原生羽毛中。这表明飞行羽毛的状况和蜕皮的时间很可能是选择生命第一年推迟迁徙的重要因素。与飞行羽毛蜕变无关的其他因素，似乎已引起成年羽毛成年，这些成年人完成了飞行中的羽毛蜕变，但脂肪含量低和/或不完整的羽化羽毛。我们没有发现免疫系统减弱，血液寄生虫高负荷或高压力水平的证据，可以解释这种不良的迁徙条件并因此而过度消费。我们的数据表明，我们的成年年龄段既包括推迟初次繁殖直到两岁的年轻个体，也包括具有生殖经验而没有繁殖机会的性成熟个体。首次繁殖的繁殖倾向和年龄都鲜为人知，但决定种群增长的关键人口参数。这项研究表明，有可能通过非繁殖地区的标记捕获来估计它们，这值得进一步研究。