Data from: Individual shifts toward safety explain age-related foraging distribution in a gregarious shorebird

Although age-related spatial segregation is ubiquitous, the underlying mechanisms are poorly understood. Here we aim to elucidate the processes behind a previously established age-related foraging distribution of red knots (Calidris canutus canutus) in their main wintering area in West Africa (Banc d’Arguin, Mauritania). Based on 10 years of observations of 1232 uniquely colour-ringed individuals of 1 to 18+ years old, we examined whether the observed age-related foraging distribution resulted from (1) spatial differences in mortality or (2) age-related shifts in habitat use. Using multi-state capture-recapture modelling, we showed that with age foraging red knots moved away from the shoreline, i.e. to areas with fewer surprise attacks by raptors. Considering uncertainties in the subjective gradient in predation danger with increasing distance from shore (as assessed from correlations between vigilance and distance from shore in foraging birds), we applied two different danger zone boundaries, at 40 and 500 m from shore. Between years, red knots had a much higher chance to move from the dangerous nearshore area to the ‘safe’ area beyond (71-78% and 26% for 40 m and 500 m danger zone boundary, respectively), than vice versa (4% and 14%). For neither danger zone boundary value did we find differences in annual mortality for individuals using either dangerous or safe zone, so the move away from the shore with age is attributed to individual careers rather than differential mortality. We argue that longitudinal studies like ours will reveal that ontogenetic shifts in habitat use are more common than so far acknowledged.

尽管年龄相关的空间分异现象普遍存在，但其背后的潜在机制仍未得到充分阐释。本研究旨在解析西非毛里塔尼亚阿尔金滩（Banc d’Arguin, Mauritania）——红腹滨鹬（Calidris canutus canutus）的主要越冬地——中，此前已被证实的年龄相关觅食分布格局的形成机制。本研究基于10年间对1232只佩戴唯一标识彩色脚环、年龄跨度为1至18岁以上个体的观测数据，检验了观测到的年龄相关觅食分布格局是否由以下两种因素导致：(1) 不同空间区域的死亡率差异，或(2) 栖息地使用随年龄发生的转变。通过多状态捕获重捕模型（multi-state capture-recapture modelling）分析，我们发现随着年龄增长，觅食中的红腹滨鹬会逐渐远离海岸线，即迁移至猛禽突袭概率更低的区域。考虑到随离岸距离增加，捕食风险的主观梯度存在不确定性（该梯度通过觅食鸟类的警戒行为与离岸距离的相关性进行评估），我们分别采用了离岸40米和500米的两种不同危险区边界划分标准。年度间的迁移结果显示，红腹滨鹬从近岸危险区域迁移至外侧“安全”区域的概率（分别为40米危险区边界下的71%-78%，以及500米危险区边界下的26%）远高于反向迁移的概率（分别为4%和14%）。在两种危险区边界划分标准下，我们均未发现栖息于危险区域与安全区域的个体间年死亡率存在显著差异，因此红腹滨鹬随年龄远离海岸线的行为，应归因于个体自身的栖息行为随龄转变，而非不同区域的死亡率差异。我们认为，类似本研究的长期纵向追踪研究将证实，栖息地使用的随龄转变现象比目前学界所认知的更为普遍。