Dissertation Research: Evolution of Altitudinal Migration in Tropical Birds: A Test of Three Hypotheses

许多动物每年都会进行壮观的迁徙，距离从几公里到跨极航行不等。现在人们对这些动物所遵循的路线以及启动迁徙行为的线索已经有了很大了解。然而，为什么一些物种迁徙，而另一些物种从不迁徙，这仍然是一个谜。人们提出了许多可能性，但一直缺乏严格的测试。该项目将测试对三种假说的预测，以解释为什么一些但不是所有吃水果的热带鸟类在繁殖后迁徙到山上繁殖，并在繁殖后返回山坡：水果可获得性假说(候鸟追踪水果的季节性可获得性)；节肢动物可获得性假说(候鸟追踪节肢动物的季节性可获得性，节肢动物是它们饮食中次要但关键的组成部分)；以及巢捕食性假说(鸟类迁徙上山以躲避以卵和幼鸟为食的高密度捕食者)。这项研究将在哥斯达黎加大西洋一侧受保护的热带森林中沿着海拔梯度进行，那里的鸟类活动模式以及鸟类和植物的多样性异常众所周知。这项工作将集中在一种典型的、数量丰富的物种--白皱纹山雀(Corapipo Altera)上。这些假设的测试包括确定(A)有什么食物可用，(B)人造象以什么为食，以及(C)捕食者在一年中不同的海拔和不同的时间攻击人工巢的频率。除了揭开一个著名的生物学谜团之外，了解鸟类迁徙的原因应该会阐明热带生物栖息地变化的后果。这项工作最终应该表明人类造成的栖息地改变的可能影响，包括气候变化和砍伐森林，在这些受威胁的景观中。

Many animals undertake spectacular annual migrations, across distances ranging from only a few kilometers to transpolar voyages. A great deal is now known about the routes these animals follow, and the cues that initiate migratory behavior. However, why some species migrate and others never do has remained a mystery. Many possibilities have been suggested, but rigorous tests have been lacking. This project will test predictions of three hypotheses for why some but not all fruit-eating tropical birds migrate up mountain ranges to breed, and back downhill after breeding: the fruit availability hypothesis (migratory birds track seasonal availability of fruits); the arthropod availability hypothesis (migratory birds track seasonal availability of arthropods, a minor but critical component of their diets); and the nest predation hypothesis (birds migrate uphill to escape high densities of predators that feed upon eggs and fledglings). The research will be conducted along an elevational gradient in protected tropical forest on the Atlantic side of Costa Rica, where patterns of bird movement and both bird and plant diversity are unusually well-known. The work will focus on one typical and abundant species, the white-ruffed manakin (Corapipo altera). Tests of the hypotheses involve determining (a) what foods are available, (b) what manakins feed upon, and (c) how often predators attack artificial nests, at different elevations and at different times of year. Beyond shedding light on a renowned biological mystery, understanding why birds migrate should illuminate the consequences of habitat variation for tropical organisms. This work ultimately should suggest the likely impacts of human-caused habitat alterations, including climate change and deforestation, in these threatened landscapes.