Site Fidelity and Individual Variation in Winter Location in Partially Migratory European Shags

In partially migratory populations, individuals from a single breeding area experience a range of environments during the non-breeding season. If individuals show high within- and among- year fidelity to specific locations, any annual environmental effect on individual life histories could be reinforced, causing substantial demographic heterogeneity. Quantifying within- and among- individual variation and repeatability in non-breeding season location is therefore key to predicting broad-scale environmental impacts on the dynamics of partially migratory populations. We used field resightings of colour-ringed adult European shags known to have bred on the Isle of May, Scotland, to quantify individual variation and repeatability in winter location within and among three consecutive winters. In total, 3797 resightings of 882 individuals were recorded over 622 km of coastline, including the Isle of May. These individuals comprised over 50% of the known breeding population, and encompassed representative distributions of ages and sexes. The distances from the Isle of May at which individuals were resighted during winter varied substantially, up to 486 km and 136 km north and south respectively and including the breeding colony on the Isle of May. However, resighting distances were highly repeatable within individuals; within- and among-winter repeatabilities were >0.72 and >0.59 respectively across the full September-March observation period, and >0.95 and >0.79 respectively across more restricted mid-winter periods. Repeatability did not differ significantly between males and females or among different age classes, either within or among winters. These data demonstrate that the focal shag population is partially migratory, and moreover that individuals show highly repeatable variation in winter location and hence migration strategy across consecutive winters. Such high among-individual variation and within-individual repeatability, both within and among winters, could lead to substantial life history variation, and therefore influence population dynamics and future conservation management strategies.在部分迁徙种群中，同一繁殖区域内的个体在非繁殖季会经历多样化的环境条件。若个体对特定位点表现出较高的年内部及年间忠实性，则年度环境对个体生活史的影响会被强化，进而引发显著的种群统计异质性。因此，量化非繁殖季位点选择的个体内与个体间变异及可重复性，是预测大范围环境变化对部分迁徙种群动态影响的关键所在。本研究通过对已知在苏格兰五月岛(Isle of May)繁殖的成年佩戴彩色环志的欧洲鸬鹚(European shags)进行野外重识别(resighting)记录，量化了连续三个冬季内及冬季间的冬季位点选择的个体变异与可重复性。研究共计在包括五月岛在内的622千米海岸线上，记录到882只个体的3797条重识别记录。这些个体占已知繁殖种群总数的50%以上，且涵盖了具有代表性的年龄与性别分布。个体冬季重识别位点距五月岛的距离差异显著，向北最远可达486千米，向南最远可达136千米，同时也包括五月岛的繁殖群区域。不过，个体内的重识别距离具有极高的可重复性：在完整的9月至次年3月观测周期内，个体内及年间可重复性分别大于0.72和0.59；而在更严格限定的隆冬观测期内，二者分别大于0.95和0.79。无论在单个冬季还是多个冬季间，雄性与雌性之间以及不同年龄组之间的可重复性均无显著差异。这些数据表明，本研究聚焦的鸬鹚种群属于部分迁徙种群，且个体在连续冬季的冬季位点选择乃至迁徙策略均表现出高度可重复的变异。这种冬季内与冬季间显著的个体间变异及个体内可重复性，可能会引发显著的生活史变异，进而影响种群动态及未来的保护管理策略。