Data from: How many routes lead to migration? Comparison of methods to assess and characterise migratory movements

1. Decreasing rate of migration in several species as a consequence of climate change and anthropic pressure, together with increasing evidence of space-use strategies intermediate between residency and complete migration, are very strong motivations to evaluate migration occurrence and features in animal populations. 2. The main goal of this paper was to perform a relative comparison between methods for identifying and characterising migration at the individual and population level on the basis of animal location data. 3. We classified 104 yearly individual trajectories from five populations of three deer species as migratory or non-migratory, by means of three methods: seasonal home range overlap, spatio-temporal separation of seasonal clusters, and the Net Squared Displacement (NSD) method. For migratory cases, we also measured timing and distance of migration and residence time on the summer range. Finally, we compared the classification in migration cases across methods and populations. 4. All methods consistently identified migration at the population level, i.e., they coherently distinguished between complete or almost complete migratory populations and partially migratory populations. However, in the latter case, methods coherently classified only about 50% of the single cases, i.e. they classified differently at the individual-animal level. We therefore infer that the comparison of methods may help point to ‘less-stereotyped’ cases in the residency-to-migration continuum. For cases consistently classified by all methods, no significant differences were found in migration distance, or residence time on summer ranges. Timing of migration estimated by NSD was earlier than by the other two methods, both for spring and autumn migrations. 5. We suggest 3 steps to identify improper inferences from migration data, and to enhance understanding of intermediate space-use strategies. We recommend: a) classifying migration behaviours using more than one method, b) performing sensitivity analysis on method parameters to identify the extent of the differences, and c) investigating inconsistently classified cases as these may often be ecologically interesting (i.e., less-stereotyped migratory behaviours).1. 受气候变化与人为活动压力影响，多个物种种群的迁徙速率持续降低；与此同时，越来越多证据显示存在介于定居（residency）与完全迁徙（complete migration）之间的空间利用策略（space-use strategies），这两点均为评估动物种群的迁徙发生情况及其特征提供了极强的研究动因。2. 本研究的核心目标是基于动物定位数据，开展用于个体与种群层面的迁徙识别与特征刻画方法的对比分析。3. 我们借助三种方法，将三种鹿科物种的五个种群共104条年度个体轨迹划分为迁徙或非迁徙个体：分别为季节家域重叠法、季节聚类时空分离法，以及净平方位移（Net Squared Displacement，NSD）方法。针对被划分为迁徙的个体，我们还测定了其迁徙时间、迁徙距离以及夏季活动区的停留时长。最后，我们对比了不同方法与种群间的迁徙个体分类结果。4. 所有方法均能在种群层面一致识别出迁徙模式，即可清晰区分完全或近乎完全迁徙种群与部分迁徙种群。但针对后者（即部分迁徙种群）时，各方法仅能对约50%的单个个体做出一致分类，即不同方法在个体层面的分类结果存在差异。因此我们推断，对不同方法的对比分析，有助于识别定居-迁徙连续谱中“非典型”的迁徙案例。针对所有方法均一致分类的迁徙个体，其迁徙距离与夏季活动区停留时长均无显著差异。无论是春季还是秋季迁徙，通过NSD方法估算的迁徙时间均早于另外两种方法。5. 我们提出了三步流程，用于识别迁徙数据中的不当推断，并加深对中间空间利用策略的认知。具体建议如下：a）采用多种方法开展迁徙行为分类；b）对方法参数开展敏感性分析，以明确差异程度；c）针对分类结果不一致的个体开展研究，此类案例往往具备较高的生态学研究价值（即非典型迁徙行为。