Data from: Carry-over effects on the annual cycle of a migratory seabird: an experimental study

Long-lived migratory animals must balance the cost of current reproduction with their own condition ahead of a challenging migration and future reproduction. In these species, carry-over effects, which occur when events in one season affect the outcome of the subsequent season, may be particularly exacerbated. However, how carry-over effects influence future breeding outcomes and whether (and how) they also affect behaviour during migration and wintering is unclear.
Here we investigate carry-over effects induced by a controlled, bidirectional manipulation of the duration of reproductive effort on the migratory, wintering and subsequent breeding behaviour of a long-lived migratory seabird, the Manx shearwater Puffinus puffinus. By cross-fostering chicks of different age between nests, we successfully prolonged or shortened by ∼25% the chick-rearing period of 42 breeding pairs. We tracked the adults with geolocators over the subsequent year and combined migration route data with at-sea activity budgets obtained from high-resolution saltwater-immersion data. Migratory behaviour was also recorded during non-experimental years (the year before and/or two years after manipulation) for a subset of birds, allowing comparison between experimental and non-experimental years within treatment groups.
All birds cared for chicks until normal fledging age, resulting in birds with a longer breeding period delaying their departure on migration; however, birds that finished breeding earlier did not start migrating earlier. Increased reproductive effort resulted in less time spent at the wintering grounds, a reduction in time spent resting daily and a delayed start of breeding with lighter eggs and chicks and lower breeding success the following breeding season. Conversely, reduced reproductive effort resulted in more time resting and less time foraging during the winter, but a similar breeding phenology and success compared with control birds the following year, suggesting that ‘positive’ carry-over effects may also occur but perhaps have a less long-lasting impact than those incurred from increased reproductive effort.
Our results shed light on how carry-over effects can develop and modify an adult animal's behaviour year-round and reveal how a complex interaction between current and future reproductive fitness, individual condition and external constraints can influence life-history decisions.

长距离迁徙的长寿动物在面临高难度迁徙与未来繁殖任务前，需权衡当前繁殖的代价与自身身体状态。此类物种中，当某一季节的事件影响后续季节的繁殖结果时，便会产生携带效应（carry-over effects），且该效应在这类物种中往往更为显著。然而，携带效应如何影响后续繁殖结果，以及其是否（以及如何）影响迁徙与越冬期间的行为，目前仍不明确。
本研究针对长距离迁徙海鸟——曼克斯剪水鹱（Puffinus puffinus），通过对繁殖投入时长进行双向可控操控，探究其携带效应对迁徙、越冬及后续繁殖行为的影响。研究人员通过交叉抚育不同日龄的雏鸟，成功将42对繁殖个体的育雏时长延长或缩短了约25%。后续一年中，我们利用地理定位器（geolocators）对成鸟进行追踪，并结合迁徙路线数据与从高分辨率海水浸没数据中获取的海上活动时间分配信息。此外，针对部分个体，研究人员还记录了其非实验年份（操控前一年及/或操控后两年）的迁徙行为，从而可在同一处理组内对比实验与非实验年份的行为差异。
所有受试个体均将雏鸟抚育至正常离巢日龄，结果显示：育雏周期更长的个体，其迁徙出发时间也相应延迟；但提前结束繁殖的个体，其迁徙启动时间并未提前。繁殖投入增加的个体，其在越冬地停留的时间更短，每日休息时长也有所减少；且在下一个繁殖季，其繁殖启动时间延迟，所产的卵与雏鸟体重更轻，繁殖成功率也更低。反之，繁殖投入减少的个体，越冬期间休息时长更长、觅食时长更短，但其下一个繁殖季的繁殖物候（breeding phenology）与繁殖成功率与对照组个体并无显著差异，这表明“正向”携带效应同样可能存在，但其持续时长或许不如繁殖投入增加所引发的携带效应长久。
本研究结果阐明了携带效应如何形成并全年改变成体动物的行为，同时揭示了当前与未来繁殖适合度（reproductive fitness）、个体身体状态以及外部限制因素之间的复杂交互作用，如何影响动物的生活史决策（life-history decisions）。