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.

长寿迁徙动物必须权衡当前繁殖的代价，以及应对具有挑战性的迁徙与未来繁殖所需维持的自身身体状态。在这类物种中，当某一季节的事件影响后续季节的结果时所产生的携带效应（carry-over effects），其影响可能会被进一步加剧。然而，携带效应如何影响未来的繁殖结局，以及它们是否（以及如何）影响迁徙和越冬期间的行为，目前仍不明确。

本研究以长寿迁徙海鸟曼岛鹱（*Puffinus puffinus*）为研究对象，通过对繁殖投入时长进行双向可控操控，探究其对该物种迁徙、越冬及后续繁殖行为的携带效应。我们通过在不同巢穴间交叉抚育不同日龄的雏鸟，成功将42对繁殖个体的育雏期延长或缩短了约25%。在后续一年中，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数据。此外，我们还记录了部分个体在非实验年份（操控前一年和/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年份与非实验年份的行为差异。

所有受试个体均抚育雏鸟至正常离巢日龄，结果显示：育雏期更长的个体，其迁徙出发时间会延迟；但提前完成繁殖的个体，并未更早启动迁徙。更高的繁殖投入会导致个体在越冬地停留的时间减少，每日休息时间缩短，且次年繁殖季的繁殖起始时间延迟，所产鸟蛋和雏鸟体重更轻，繁殖成功率更低。反之，降低繁殖投入则会使个体在越冬期间的休息时间增加、觅食时间减少，但次年的繁殖物候与繁殖成功率与对照组个体无显著差异，这表明“正向”的携带效应同样可能存在，但其影响时长或许不如高繁殖投入所引发的效应持久。

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、个体身体状况以及外部约束之间的复杂相互作用，如何影响动物的生活史决策。