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.

长距离迁徙动物必须权衡当前繁殖的代价与应对挑战性迁徙及未来繁殖所需的自身身体状况。在这类物种中，携带效应（carry-over effects）——即某一季节的事件会影响后续季节的繁殖结果——的影响可能尤为突出。然而，携带效应如何影响后续繁殖结果，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。为此，我们以长寿命迁徙海鸟曼克斯剪水鹱（Manx shearwater，*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。我们通过在不同巢间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%。我们在后续一年中借助地理定位器（geolocators）追踪成鸟，并将迁徙路线数据与通过高分辨率海水浸没数据获取的海上活动时间分配相结合。我们还对部分个体在非实验年份（操控实施前一年和/或操控后两年）的迁徙行为进行了记录，从而可以在处理组内对比实验年份与非实验年份的行为差异。所有个体均将雏鸟抚育至正常离巢龄，因此育雏周期更长的个体其迁徙出发时间延后；但提前完成繁殖的个体并未更早开启迁徙。更高的繁殖投入会导致个体在越冬地停留的时间减少，每日休息时长缩短，且次年繁殖季的繁殖启动时间延后，同时所产鸟蛋和雏鸟体重更轻，繁殖成功率更低。与之相反，降低繁殖投入的个体在越冬期间休息时长更长、觅食时长更短，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’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带来的携带效应持久。本研究阐明了携带效应如何形成并全年改变成年动物的行为模式，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。