One of the most amazing aspects of migration is how birds find their way over long distances. Many species are capable of migrating between exactly the same breeding and wintering places year after year, even if these places lie thousands of kilometres apart on different continents. Young birds migrating alone can find their own way to the usual wintering areas for their species, and back to their natal areas the following spring. Some pelagic seabirds wander widely over the oceans, yet each year return unfailingly to their own particular nesting islands. Great Shearwaters Puffinus gravis, for example, nest on the isolated Tristan da Cunha islands, lying at 40°S in the South Atlantic and more than 2000 km from Africa, the nearest continent. In the non-breeding season these birds migrate northward in their millions, ranging over large parts of the North Atlantic. But they return each year with pinpoint accuracy to their tiny breeding islands, which are spread over only 45 km of ocean, and individuals occupy the same nest burrows from year to year, often lying within a metre of those of other individuals. These and other seabirds that migrate long, overwater distances to small oceanic islands must surely be among the greatest of animal navigators, possessing extremely accurate orientation mechanisms.
Like human navigators, birds and other animals can find their way over long distances only with the aid of a reliable reference system by which to navigate. Research has confirmed that birds use at least two types of system, based on geomagnetic and celestial cues (the sun by day and the stars at night) respectively. However, a compass is of little value to a migratory bird unless it 'knows' beforehand - either by inheritance or experience - what course it needs to take. The mechanisms of bird orientation and navigation are discussed in Chapter 9.
The timing of bird migration is equally remarkable. Many long-distance bird migrants arrive at their nesting or wintering places every year at around the same date. This implies the existence in the birds of precise timing mechanisms that, in response to external stimuli, trigger migration at about the same dates each year and maintain it for long enough to allow the bird to cover the distance required. Such mechanisms ensure that individuals arrive in their nesting areas as conditions become suitable for breeding and leave before conditions deteriorate and affect survival. The relatively small variations in timing that occur from year to year are mainly associated with variations in prevailing weather or food supplies (Chapter 12).
A third adaptation that facilitates seasonal migration is the ability of birds at appropriate times of year to accumulate large body reserves (mostly fat) to fuel the flights (Chapter 5). Small birds that cross large areas of sea or desert in which they cannot feed are able to double their usual weight beforehand through fuel deposition, and some species also reduce the mass of other body organs not directly concerned with migration, thus reducing the overall energy needs of the journey. The seasonal changes in body composition that occur in migratory birds are some of the most extreme of the animal world. Birds are also unusual in the speed and efficiency with which they can convert the fatty acids in fuel reserves to the energy needed to power the wings.
The migratory lifestyle requires that periods of movements are integrated with other events in the birds' annual cycle, especially breeding or moult. In most bird species, these events normally occur at different times of year, with minimal overlap between them. Because the act of breeding requires that birds remain within restricted localities, it is obvious that individuals cannot breed and migrate at the same time. And because feather replacement can temporarily reduce flight efficiency, it is also desirable that moult and migration are separated as much as possible. Studies of the annual cycles of birds, and the physiological control of migration within these cycles, are discussed in Chapters 11 and 12.
An interesting aspect of bird migration concerns the extent to which individuals are pre-programmed by inheritance to do the right things at the right times of year. Without innate programming, an individual would have little sense of when to migrate, in which direction to fly or for how long. Nor would it know when on its journey to do specific things, such as change direction or accumulate extra body reserves in preparation for a long sea-crossing. All these aspects require an endogenous schedule which promotes particular kinds of behaviour at appropriate times of year or stages in a journey. This inherent component of some bird movements adds an additional fascination to study of the controlling mechanisms (Chapter 12).
Yet despite being partly under genetic control, migration patterns among birds show great flexibility and facility for rapid change (Chapter 20). Many bird families contain both migratory and non-migratory populations, showing little phylo-genetic constraint on the development of migratory behaviour. Within species, changes in migratory patterns are presumed to have occurred repeatedly through the Pleistocene glacial cycles and, more strikingly, even in recent decades, as particular populations have become more sedentary, or shortened their migrations, in apparent response to climate warming (Chapter 21). Further understanding of the evolution of migration systems can be inferred from present distribution and movement patterns, as well as from palaeontological and molecular evidence (Chapters 22 and 23).
To accommodate a long-distance migratory lifestyle, participants must be able to live in two or more different parts of the world, often on different continents. They must often occupy somewhat different habitats and climatic regimes, deal with different foods, and exist within different communities, filling distinct niches in both their summer and winter homes. Such split lives have consequences that a sedentary lifestyle does not. In particular, the population levels of migratory birds can be influenced by conditions in breeding, migration and wintering areas, and conditions experienced in wintering or migration areas can affect subsequent survival and breeding success (Chapter 26). Recent widescale declines in the numbers of many migratory species, from both the Eurasian-African and the North American-South American bird migration systems, have stimulated research into what limits the population sizes of migrants, and whether the limitation occurs primarily in wintering, breeding or migration areas (Chapters 26-28).
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