How Migrants Get There: Migratory Performance and Orientation

摘要

Migratory animals show a suite of adaptations to cope with their journeys. These include not only morphological features for efficient locomotion and storage of energy but also behavioral adjustments to exploit winds and currents or to avoid drift caused by moving fluids. Migration strategies across locomotory modes can be analyzed in the context of optimality models, using some general principles concerning migration range and selection criteria. Comparisons of model predictions with natural behavior help researchers understand the selection pressures that underlie migration strategies. We give examples of typical migration speeds and distances for animals using different locomotion models. Successful migration also requires accurate orientation and/or navigation between distant areas for reproduction and survival. Animals can use a suite of different compasses, which may be cross-calibrated or integrated for direction finding, depending on the geographical and ecological situation, and may be used with an endogenous clock for time compensation.nnLong-distance migration is the process of transportation between widely separated areas, typically divided in cycles of energy accumulation (fueling) and movement toward the goal. These cycles are repeated until the goal is reached. At this level of analysis, migration is deceptively simple. In reality, however, migration is a complex process that offers an ideal study system for the evolutionary biologist, as it involves specific adaptations involving morphology and physiology for efficient energy accumulation and locomotion, and behavioral adaptations for optimal use of external factors (winds, currents, orientation cues). Together these adaptations define the migration syndrome (i.e., the suite of characters that helps the animal to migrate more effectively than a resident). The experimental and modeling toolkit available to study migration has expanded dramatically in recent years, involving cross-disciplinary approaches and methods such as molecular genetics, isotope analyses, magnetic resonance imaging, ultrasound, electromagnetic coils, and wind tunnels.nnThe physical nature of locomotion, and hence of migration, makes it amenable to optimality analysis, as evidenced by the development of migration theory over the past two decades (Alerstam and Hedenström 1998). Here we discuss the opportunities and constraints provided by different media and modes of migration, at stages including fueling, locomotion, and orientation. In this article we will restrict ourselves to animals whose mode of locomotion is swimming, flying, or running. Migration also occurs in organisms at other (smaller) scales using other types of locomotion, but we believe the governing principles will nevertheless be similar. Our presentation is biased toward birds, partly because of our own background but also because research on bird migration processes has made more progress than that on other migratory organisms. For a thorough discussion on the definition of migration, we recommend Dingle (1996) and Dingle and Drake (2007).