Data from: Ecological forensics: using single point stable isotope values to infer seasonal schedules of animals after two diet switches

Animals adjust to seasonal challenges in physical, behavioural and spatial ways. Such adjustments are commonly associated with diet changes that often can be characterised isotopically.
We introduce the ‘double diet switch model’, with which the occurrence and timing of two subsequent diet switches of an individual animal can be traced with a single sample assayed for stable isotopes. We demonstrate the model for Sanderling, Calidris alba, a small shorebird that migrates from the Nearctic tundra breeding grounds to the intertidal flats of the Wadden Sea; during this migration some birds may stage in the North Atlantic areas.
The ‘double diet switch model’ successfully predicted the occurrence and timing of two diet switches in 59 Sanderlings captured in the Wadden Sea in July–September. Excluding birds that likely had over-summered at North Atlantic staging areas, the model predicted that Sanderlings departed from the Arctic on 13 July (range: 9–17 July), had a staging duration of 18·6 days in the North Atlantic, and arrived in the Wadden Sea on 1 August (31 July–1 August).The estimated mean Arctic departure dates coincided with the mean hatching date, suggesting that many individuals failed to produce young or left the care to a partner. Estimated mean arrival date matched the main arrival period in the Wadden Sea obtained from observation data. In this study we did not use lipid-free tissues, which may bias model predictions. After correcting for lipid components, the estimated departure date was 11 days later and the staging duration 8·5 days shorter, while arrival date was similar.
The ‘double diet switch model’ successfully identified the occurrence and timing of two subsequent diet switches. The ‘double diet switch model’ will not only apply to switches between three isotopic levels (as in the case study on Sanderling) but also to scenarios where the second switch reverses to the initial isotopic level. Due to this general applicability, the model can be adapted to a wide range of taxa and situations. Foreseeable applications include changes in habitat and food type, ontogenetic development or drastic phenotypic changes such as the metamorphosis in insects and amphibians.

动物会通过生理、行为及空间维度的调整来适应季节性环境挑战。这类调整通常与饮食变化相关，而饮食变化往往可通过同位素特征进行表征。
本研究提出“双饮食转换模型（double diet switch model）”，通过单份稳定同位素（stable isotopes）检测样本，即可追溯单个动物的两次连续饮食转换事件及其发生时间。我们以三趾鹬（Sanderling，Calidris alba）为例对该模型进行验证：该物种为小型涉禽，繁殖于新北极冻原地带，迁徙至瓦登海的潮间带滩涂越冬，部分个体在迁徙途中会在北大西洋区域停留休整。
“双饮食转换模型”成功预测了7月至9月间在瓦登海捕获的59只三趾鹬的两次饮食转换事件及其发生时间。剔除可能在北大西洋停留休整地越夏的个体后，模型预测三趾鹬的北极繁殖地平均离开时间为7月13日（范围：7月9日至17日），在北大西洋的停留时长为18.6天，并于8月1日（7月31日至8月1日）抵达瓦登海。
估算得到的北极繁殖地平均离开时间与平均孵化时间重合，这表明多数个体未能成功繁育后代，或是将育雏任务交由配偶完成。估算得到的平均抵达时间与观测数据中三趾鹬抵达瓦登海的主要时段相吻合。本研究未使用去脂组织，这可能会对模型预测结果产生偏倚。在对脂质成分进行校正后，估算的北极离开时间延后了11天，停留时长缩短了8.5天，而抵达时间则无明显变化。
“双饮食转换模型”可有效识别两次连续饮食转换事件的发生与时间。该模型不仅适用于三种同位素水平间的转换（如三趾鹬的案例研究），还可应用于第二次转换回归至初始同位素水平的场景。得益于这种广泛适用性，该模型可适配绝大多数类群与研究场景。其可预见的应用场景包括栖息地与食物类型的转变、个体发育过程，或是昆虫、两栖类的变态发育等剧烈表型变化。