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. 受气候变化与人类活动压力影响，多个物种种群的迁徙率持续下降；同时越来越多证据表明，动物存在介于定居行为与完全迁徙之间的中间型空间利用策略。上述两点为评估动物种群的迁徙发生情况与特征提供了极强的研究动机。
2. 本研究的核心目标为：基于动物定位数据，对用于个体与种群尺度迁徙识别与特征描述的各类方法开展对比分析。
3. 本研究依托三种方法，将3种鹿类5个种群的104条年度个体移动轨迹划分为迁徙型与非迁徙型：分别为季节性家域重叠率分析法、季节性集群时空分离分析法，以及净平方位移（Net Squared Displacement, NSD）法。针对判定为迁徙型的轨迹，本研究同时测算其迁徙时间、迁徙距离以及夏季活动区的停留时长。最终，本研究对不同方法与种群间的迁徙分类结果开展了对比。
4. 所有方法在种群尺度上均能稳定识别迁徙特征，即可一致性地区分完全/近乎完全迁徙种群与部分迁徙种群。但针对部分迁徙种群，各方法仅能对约50%的个体案例达成一致分类，即个体尺度的分类结果存在分歧。据此可推断，方法对比研究有助于定位定居-迁徙连续体中的非典型案例。对于所有方法均达成一致分类的案例，其迁徙距离与夏季活动区停留时长均无显著差异。通过NSD法估算的迁徙时间（包括春季与秋季迁徙）早于另外两种方法。
5. 本研究提出3个步骤，用于识别迁徙数据中的不当推论，并深化对中间型空间利用策略的认知：其一，采用多种方法开展迁徙行为分类；其二，对方法参数开展敏感性分析，以明确分类差异的程度；其三，针对分类结果不一致的案例开展深入研究——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）。