An experimental evaluation of the effects of geolocator design and attachment method on between-year survival on whinchats Saxicola rubetra

Data from location logging tags have revolutionised our understanding of migration ecology, but methods of tagging that do not compromise survival need to be identified. We compared resighting rates for 156 geolocator-tagged and 316 colour ringed-only whinchats on their African wintering grounds after migration to and from eastern Europe in two separate years. We experimentally varied both light stalk length (0, 5 and 10 mm) and harness material (elastic or non-elastic nylon braid tied on, leg-loop ‘Rappole’ harnesses) in the second year using a reasonably balanced design (all tags in the first year used an elastic harness and 10 mm light stalk). Tags weighed 0.63 g (0.01 SE), representing 4.1% of average body mass. There was no overall significant reduction in between-year resighting rate (our proxy for survival) comparing tagged and untagged birds in either year. When comparing within tagged birds, however, using a tied harness significantly reduced resighting rate by 53% on average compared to using an elastic harness (in all models), but stalk length effects were not statistically significant in any model considered. There was no strong evidence that the fit (relative tightness) or added tag mass affected survival, although tied tags were fitted more tightly later in the study, and birds fitted with tied tags later may have had lower survival. Overall, on a precautionary principle, deploying tags with non-elastic tied harnesses should be avoided because the necessary fit, so as not to reduce survival, is time-consuming to achieve and does not necessarily improve with experience. Geolocator tags of the recommended percentage of body mass fitted with elastic leg-loop harnesses and with short light stalks can be used without survival effects in small long-distance migrant birds.

定位记录标签（location logging tags）所产生的数据极大地革新了我们对迁徙生态学的认知，但亟需明确不会损害个体生存的标记方法。本研究在两个独立年度中，对往返东欧迁徙后在非洲越冬地的156只佩戴地理定位器（geolocator）标记的欧亚石䳭（whinchat）以及316只仅佩戴彩色脚环的欧亚石䳭的重见率进行了比较。第二年的实验中，我们采用合理均衡的实验设计，对光感杆长度（0、5和10毫米）以及背带材质（系缚式弹性/非弹性尼龙编织带、腿环式"Rappole"背带）两个变量开展调控；第一年所有标记装置均采用弹性背带与10毫米光感杆。标记装置的平均重量为0.63克（标准误SE=0.01），占个体平均体重的4.1%。无论在哪个年度，相较于未标记个体，标记个体的年际重见率（本研究中将其作为生存状况的替代指标）均未出现显著的整体下降。然而在标记个体内部的对比中，相较于弹性背带，使用系缚式背带会使重见率平均降低53%（所有分析模型均验证了该结果），但光感杆长度在所有被考量的模型中均未表现出显著的统计学效应。尽管本研究后期系缚式标记装置的安装紧密度有所提升，且后期佩戴系缚式标记的个体存活率可能更低，但尚无充分证据表明标记装置的贴合度（相对松紧程度）或额外的标记重量会对生存状况产生影响。总体而言，基于预防性原则，应避免使用搭载非弹性系缚式背带的标记装置，因为要达到不降低存活率的必要贴合度既耗时耗力，且未必能随着操作经验的积累而得到改善。对于小型长途迁徙鸟类，使用符合推荐体重占比的地理定位器标签，并搭配弹性腿环式背带与短光感杆，不会对其生存状况产生负面影响。