Introduction
Animals move from place to place. In fact, old school text books herald mobility as a singular defining feature of animals, one that consistently sets them apart from our other earthly colleagues, the plants. Although branches may sway in the breeze and twigs may turn toward the sun or stretch out tiny tendrils to grasp a support, we do not consider plants to be mobile. Plants colonize the reaches of our planet only through dispersal, by sending the next generation farther afield than the previous one. Animals, on the other hand, move regularly to find food or mates, to escape from predators or to find a sanctuary for rest or shelter. Sometimes animals move for apparently no reason at all.
Certainly not all animal movement is migration. So where do we set the boundaries of our definition? We will find that they encompass both direction and purpose. And for the animals that do migrate, that purpose is so deeply entrenched in their being that it has allowed evolution to mold every aspect of their anatomy, physiology, ecology and behavior.
Animal migration entails a two-way journey between places that differ significantly in the resources they offer. There must be a distinct advantage to the voyage, because costs are undoubtedly incurred in getting there. Moreover, migration usually occurs in a fixed direction and is associated with a predictable cycle. We are most familiar with seasonal migration, a widespread phenomenon marked by an exodus of animals in autumn. What comes to mind most readily in this regard is latitudinal migration. As the seasons change, many Northern Hemisphere animals travel south; likewise, those in the Southern Hemisphere may journey north. But migration can also occur longitudinally or vertically, particularly among creatures in the world's oceans, and altitudinally, as animals move up and down mountainsides to take advantage of diverse resources at different elevations. Patchy resources may have causes other than climate, and animals migrate for those reasons as well.
Many thousands of animal species migrate. Among them are insects, fishes, frogs, reptiles, birds and mammals -- representatives of every class of vertebrate and many invertebrates as well. But birds stand out among them as the very essence of migration. Why? Because among all these creatures, only birds exhibit such a striking union of two characteristics: flight and endothermy, or "warm-bloodedness."
Studies of animals that migrate on foot, by swimming or on the wing clearly demonstrate that these modes of travel differ considerably in speed and energy consumption. Flight may be energetically expensive, but it can yield the fastest speed over land. In addition, flight allows passage over barriers that may restrict movements of other animals. Walking, on the other hand, may consume less energy than flight but it sacrifices speed, and thus distance.
Among terrestrial animals, woodland caribou (Rangifer tarandus), which migrate seasonally about 800 miles (1,300 km) between their forested winter home and spring calving areas on the tundra, travel the farthest. Endothermic animals like caribou are able to sustain a moderate pace because they can maintain the high rate of metabolism needed to fuel their activities. Terrestrial ectothermic, or "cold blooded," animals such as frogs and snakes, on the other hand, barely migrate at all. Ectotherms use their environment to raise their body temperature enough to pursue their daily routines; their greatest sacrifice is endurance. This primary limitation-by-design restricts the lowly striped whipsnake (Masticophis taeniatus) to migratory journeys of no more than about 2.5 miles (4 km). The European pool frog (Rana lessonae) fares slightly better by hopping; its migratory voyage between overwintering sites and breeding ponds measures an extremely dangerous 9 miles (15km). This is in no way intended to malign ectothermy, nor to imply that endothermy is an inherently better strategy. After all, ectotherms have certain advantages: some hibernating toads can exist quite comfortably without food for three years or more.
Endothermy is the preferred metabolic mode for migration on land, but it may not be so critical under water. Indeed, gray whales (Eschrichtius robustus -- which are endothermic mammals, of course -- travel about 5,100 miles (8,200 km) between their rich northern feeding waters and their calving grounds off the coast of Mexico's Baja California peninsula. However, female loggerhead sea turtles (Caretta caretta), which are ectothermic reptiles, migrate farther -- some 7,150 miles (11,500 km) -- as they return to their traditional sandy beaches every two to four years to lay their eggs. Even a few fish species, such as the European eel (Anguilla anguilla) and bluefin tuna (Thunnus thynnus), are capable of migrations of many thousands of miles over an extended period of time. Unlike terrestrial animals, of course, aquatic animals are suspended in a buoyant medium, which makes swimming quite cost-effective. But speed is still a major consideration for most species.
Time is of the essence in migration. The journey itself serves only to move the animal from one place to another, where it will linger until a change in the environment stimulates a return trip. A voyage that takes too much of a species' annual cycle is not practical unless the travel is no more hazardous than the destination, and this is rarely the case. Consequently, swimming and walking are usually too slow for most species to feasibly make round trips of 6,000 miles (10,000 km) or more. And most of those longer trips, including the reproductive migrations of sea turtles, are not accomplished every calendar year. Speed of travel is key, and this, of course, brings up the issue of flight.
Under favorable conditions, flight can be very swift -- perhaps 10 to 20 times or more faster than swimming or walking. For example, a 600-mile (1,000 km) journey that would take a large flying bird only a single day would require about 40 days for a swimming penguin to complete. A small terrestrial mammal would need most of the winter to make such a long voyage. Considering the distances birds cover over ground, flight is truly the most effective mode. The ruddy turnstone (Arenaria interpres), a small shorebird, can fly about 125 miles (200 km) on a single gram of fat. Emperor penguins (Aptenodytes forsteri), which walk across the ice on the first leg of their journey from breeding grounds to feeding grounds, use about 1,500 times as much energy to cover the same distance. That is why long-distance migration is most pervasive among the volant, or flying, animals -- birds, bats and insects.
Among these species, birds triumph in their migratory feats. They are indeed the experts of long-distance travel. Perhaps this is because avian flight is not merely a strategy pasted onto the anatomy of a terrestrial creature. Birds are the very epitome of flight. Over the past 150 million years, every system in their bodies has been honed to increase lift and reduce drag, to maximize power and minimize weight. Their flapping wings drive them headlong through the slipstream, fueled by turbocharged respiratory and cardiovascular systems designed to deliver oxygen to their muscles at amazingly high rates. Even waste products are so swiftly removed from their bodies that birds do not suffer the muscle pain that haunts human athletes. Many species can beat their wings continuously for days without rest; for these travelers, the length of their journey is limited only by the size of our planet.
Birds also possess innate knowledge of where they are and where they are going -- the ability to chart a course and navigate the route to its end. They have an undeniable sense of home and an unrelenting desire to return there. Passed down through evolutionary time from ancestral lineages, these tools have allowed birds to expand their distribution, reclaim lost populations and exploit a bounty of resources in faraway places. Almost 10,000
