Do American Dippers Obtain a Survival Benefit from Altitudinal Migration?

Studies of partial migrants provide an opportunity to assess the cost and benefits of migration. Previous work has demonstrated that sedentary American dippers (residents) have higher annual productivity than altitudinal migrants that move to higher elevations to breed. Here we use a ten-year (30 period) mark-recapture dataset to evaluate whether migrants offset their lower productivity with higher survival during the migration-breeding period when they occupy different habitat, or early and late-winter periods when they coexist with residents. Mark-recapture models provide no evidence that apparent monthly survival of migrants is higher than that of residents at any time of the year. The best-supported model suggests that monthly survival is higher in the migration-breeding period than winter periods. Another well-supported model suggested that residency conferred a survival benefit, and annual apparent survival (calculated from model weighted monthly apparent survival estimates using the Delta method) of residents (0.511 ± 0.038SE) was slightly higher than that of migrants (0.487 ± 0.032). Winter survival of American dippers was influenced by environmental conditions; monthly apparent survival increased as maximum daily flow rates increased and declined as winter temperatures became colder. However, we found no evidence that environmental conditions altered differences in winter survival of residents and migrants. Since migratory American dippers have lower productivity and slightly lower survival than residents our data suggests that partial migration is likely an outcome of competition for limited nest sites at low elevations, with less competitive individuals being forced to migrate to higher elevations in order to breed.

针对部分迁徙个体（partial migrants）的研究为评估迁徙的成本与收益提供了研究契机。既往研究已证实，定居型美洲河乌（sedentary American dippers）的年度繁殖成功率高于向高海拔区域迁徙繁殖的海拔迁徙型个体（altitudinal migrants）。本研究基于为期十年（涵盖30个观测周期）的标记重捕数据集（mark-recapture dataset），旨在验证迁徙型个体是否可通过占据不同栖息地的迁徙-繁殖期，或是与定居型个体共存的冬初与冬末时段的更高存活率，来弥补其较低的繁殖成功率。标记重捕模型未发现任何季节中迁徙型个体的月度表观存活率显著高于定居型个体的证据。拟合度最优的模型显示，迁徙-繁殖期的月度存活率高于冬季时段。另一支持度较高的模型则表明，定居状态可赋予存活优势：定居型美洲河乌的年度表观存活率（通过德尔塔法（Delta method）对模型加权后的月度表观存活率估算值计算得到，为0.511 ± 0.038标准误）略高于迁徙型个体（0.487 ± 0.032标准误）。美洲河乌的冬季存活率受环境条件调控：月度表观存活率随每日最大径流量升高而上升，随冬季气温降低而下降。但本研究未发现环境条件会改变定居型与迁徙型个体冬季存活率差异的证据。鉴于迁徙型美洲河乌的繁殖成功率与存活率均略低于定居型个体，本研究数据表明，部分迁徙现象大概率是低海拔区域有限巢位竞争的结果，竞争力较弱的个体被迫迁徙至高海拔区域开展繁殖。