Data from: Reproductive performance of resident and migrant males, females and pairs in a partially migratory bird

1. Quantifying among-individual variation in life-history strategies, and associated variation in reproductive performance and resulting demographic structure, is key to understanding and predicting population dynamics and life-history evolution. Partial migration, where populations comprise a mixture of resident and seasonally-migrant individuals, constitutes a dimension of life-history variation that could be associated with substantial variation in reproductive performance. However, such variation has rarely been quantified due to the challenge of measuring reproduction and migration across a sufficient number of seasonally-mobile males and females.
2. We used intensive winter (non-breeding season) resightings of colour-ringed adult European shags (Phalacrocorax aristotelis) from a known breeding colony to identify resident and migrant individuals. We tested whether two aspects of annual reproductive performance, brood hatch date and breeding success, differed between resident and migrant males, females and breeding pairs observed across three consecutive winters and breeding seasons.
3. The sex ratios of observed resident and migrant shags did not significantly differ from each other or from 1:1, suggesting that both sexes are partially migratory and that migration was not sex-biased across surveyed areas.
4. Individual resident males and females hatched their broods 6 days earlier and fledged 0.2 more chicks per year than migrant males and females on average. Resident individuals of both sexes therefore had higher breeding success than migrants.
5. Hatch date and breeding success also varied with a pair’s joint migratory strategy such that resident-resident pairs hatched their broods 12 days earlier than migrant-migrant pairs, and fledged 0.7 more chicks per year on average. However, there was no evidence of assortative pairing with respect to migratory strategy: observed frequencies of migrant-migrant and resident-resident pairs did not differ from those expected given random pairing.
6. These data demonstrate substantial variation in two key aspects of reproductive performance associated with the migratory strategies of males, females and breeding pairs within a partially migratory population. These patterns could reflect direct and/or indirect mechanisms, but imply that individual variation in migratory strategy, and variation in pairing among residents and migrants, could influence selection on migration and drive complex population and evolutionary dynamics.

1. 量化种群个体间生活史策略的差异，以及与之相关的繁殖表现差异与由此产生的种群结构，是理解和预测种群动态与生活史演化的关键。部分迁徙（Partial migration）指种群由定居个体与季节性迁徙个体混合组成，属于生活史变异的一个维度，其可能与繁殖表现的显著差异相关。然而，由于难以在足够数量的季节性移动的雌雄个体中同时测量繁殖与迁徙情况，这类变异鲜有被量化研究。
2. 我们通过对某已知繁殖群落中佩戴彩色环志的成年欧洲鸬鹚（Phalacrocorax aristotelis）开展高强度冬季（非繁殖季）重识别调查，以此区分定居个体与迁徙个体。我们针对年度繁殖表现的两个维度——雏卵孵化日期与繁殖成功率，检验了在连续三个冬季与繁殖季中观测到的定居与迁徙雄性、雌性以及繁殖配对个体之间是否存在差异。
3. 观测到的定居与迁徙鸬鹚的性别比例既未呈现显著差异，也与1:1的比例无显著偏离，这表明雌雄两性均存在部分迁徙现象，且调查区域内的迁徙行为不存在性别偏向。
4. 平均而言，定居的雌雄个体的雏卵孵化日期比迁徙个体早6天，且每年多成功抚育0.2只离巢雏鸟。因此，两种性别下的定居个体繁殖成功率均高于迁徙个体。
5. 雏卵孵化日期与繁殖成功率也随繁殖配对的联合迁徙策略发生变化：定居-定居配对的雏卵孵化日期比迁徙-迁徙配对早12天，且平均每年多成功抚育0.7只离巢雏鸟。然而，并未发现基于迁徙策略的选型配对（Assortative pairing）证据：观测到的迁徙-迁徙配对与定居-定居配对的频率，与随机配对下的预期频率并无显著差异。
6. 本数据集证实了部分迁徙种群中，雌雄个体与繁殖配对的迁徙策略与繁殖表现的两个关键维度存在显著关联。这些模式可能反映了直接和/或间接作用机制，但也暗示了迁徙策略的个体差异，以及定居者与迁徙者之间配对模式的变异，可能会影响针对迁徙的选择压力，并推动复杂的种群动态与演化过程。