Size-dependent costs of migration: migrant bird species are subordinate to residents, but only at small body sizes

Migrant species are commonly thought to be poor competitors in aggressive interactions with resident species. However, no studies have tested if this relationship is widespread. Here we compare the behavioural dominance of closely-related species of migratory and non-migratory birds, testing if migrants are consistently subordinate to resident species in aggressive contests. We compiled published behavioural dominance data involving migrant and resident congeners, gathering additional data on the body mass and migratory distance of each species. Focal species included a diverse array of birds (28 taxonomic families, 12 orders) from around the world. We found that migrant species are usually subordinate to resident species, but that this relationship disappears at larger body sizes. For smaller birds (< 500g), resident species were behaviourally dominant in 83-88% of comparisons; for larger birds (> 500g), resident species were dominant in only 25-30% of comparisons. The relative difference in body mass best predicted dominance relationships among species, with larger species dominant in 80-84% of comparisons. When migrant and resident masses were equal, however, resident species were still more likely to be dominant in smaller birds, suggesting that other factors may also contribute to the subordinate status of migrants. Overall, our results suggest that in smaller species, the evolution of migration is associated with lighter weights and other traits that compromise the competitive abilities of migrants relative to residents. In contrast, larger species appear able to evolve migration without compromising their size or competitive abilities in aggressive contests, suggesting size-dependent constraints on the evolution of migration.

学界普遍认为，迁徙物种在与居留物种的攻击性互动中竞争力偏弱。然而，目前尚无研究验证这一关联是否具有普遍性。本研究对亲缘关系相近的迁徙鸟类与非迁徙鸟类的行为优势地位开展比较研究，旨在验证迁徙物种在攻击性争斗中是否始终处于从属地位。我们收集了已发表的涉及迁徙与居留同属物种的行为优势数据，并补充收集了各物种的体重与迁徙距离信息。本研究的焦点物种涵盖全球范围内的多样鸟类类群，涉及12目、28个分类科。
研究结果显示，迁徙物种通常从属于居留物种，但这一关联在体型较大的类群中不再成立。对于体重小于500克的小型鸟类，居留物种在83%~88%的对比中占据行为优势；而对于体重大于500克的大型鸟类，居留物种仅在25%~30%的对比中占据优势。物种间的体重相对差异最能预测彼此的优势地位关系，体型更大的物种在80%~84%的对比中占据优势。
然而，当迁徙与居留物种体重相等时，小型鸟类类群中的居留物种仍更有可能占据优势，这表明其他因素或许也会导致迁徙物种处于从属地位。
总体而言，本研究结果表明，在小型物种中，迁徙的演化与较轻的体重及其他性状相关联，这些性状会削弱迁徙物种相较于居留物种的竞争力。与之相反，大型物种似乎能够在不牺牲体型或攻击性争斗竞争力的前提下演化出迁徙习性，这表明迁徙演化存在体型依赖型的约束条件。