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.
