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

许多动物每年进行一次壮观的迁徙，距离从几公里到跨越极地。对于这些动物的迁徙路线，以及引发迁徙行为的线索，我们现在已经有了很多了解。然而，为什么有些物种会迁徙，而另一些却从不迁徙，这仍然是个谜。人们提出了许多可能性，但缺乏严格的测试。这个项目将测试三种假设的预测，即为什么一些但不是所有以水果为食的热带鸟类会迁徙到山上繁殖，繁殖后又回到山下：水果可获得性假设（候鸟追踪水果的季节性可获得性）；节肢动物可得性假说（候鸟追踪节肢动物的季节性可得性，节肢动物是其饮食中次要但关键的组成部分）；以及巢捕食假说（鸟类向山上迁徙是为了躲避以鸟蛋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的受保护热带森林沿着海拔梯度进行，那里的鸟类运动模式以及鸟类和植物多样性都非常出名。这项工作将集中在一个典型的和丰富的物种，白颈侏儒鸟（Corapipo altera）。对这些假设的检验包括确定(a)有什么食物可用，(b)蜥蜴以什么为食，以及(c)在不同海拔和一年中的不同时间，捕食者攻击人工巢穴的频率。除了揭开一个著名的生物学之谜之外，了解鸟类迁徙的原因还应该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引起的栖息地改变，包括气候变化和森林砍伐，对这些受威胁的景观可能产生的影响。

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.