Data from: Light enough to travel or wise enough to stay? Brain size evolution and migratory behaviour in birds

Brain size relative to body size is smaller in migratory than in non-migratory birds. Two mutually non-exclusive hypotheses had been proposed to explain this association. On the one hand, the ‘energetic trade-off hypothesis’ claims that migratory species were selected to have smaller brains because of the interplay between neural tissue volume and migratory flight. In contrast, the ‘behavioural flexibility hypothesis’ argues that resident species are selected to have higher cognitive capacities, and therefore larger brains, to enable survival in harsh winters, or to deal with environmental seasonality. Here, I test the validity and setting of these two hypotheses using 1,466 globally distributed bird species. First, I show that the negative association between migration distance and relative brain size is very robust across species and phylogeny. Second, I provide strong support for the energetic trade-off hypothesis, by showing the validity of the trade-off among long-distance migratory species alone. Third, using resident and short-distance migratory species, I demonstrate that environmental harshness is associated with enlarged relative brain size, therefore arguably better cognition. My study provides the strongest comparative support to date for both the energetic trade-off and the behavioural flexibility hypotheses, and highlights that both mechanisms contribute to brain size evolution, but on different ends of the migratory spectrum.相较于留鸟，迁徙鸟类的相对脑体尺寸更小。目前已有两种非互斥假说被提出，用以阐释这一关联。其一为能量权衡假说（energetic trade-off hypothesis），该假说提出，由于神经组织体积与迁徙飞行之间的相互作用，自然选择青睐脑容量更小的迁徙物种。与之相对的行为灵活性假说（behavioural flexibility hypothesis）则认为，自然选择促使留鸟演化出更高的认知能力，进而拥有更大的脑容量，以帮助其在严酷冬季存活或应对环境季节性变化。本研究依托全球分布的1466个鸟类物种，对这两种假说的有效性与适用场景进行了检验。首先，研究证实，迁徙距离与相对脑体尺寸之间的负相关关系在不同物种及系统发育维度上均极为稳健。其次，仅在长距离迁徙鸟类群体中验证了能量权衡效应的存在，从而为该假说提供了强有力的支持。第三，针对留鸟与短距离迁徙鸟类群体，本研究表明环境严酷性与更大的相对脑体尺寸存在关联，即对应更强的认知能力。本研究为能量权衡假说与行为灵活性假说均提供了迄今为止最有力的比较研究证据，并指出两种机制分别在迁徙谱系的不同两端推动了脑容量的演化。