Birds are also pre-adapted for long-distance migration in ways other animals are not. One of the main advantages of flight is its speed, which is much faster than the alternatives of walking, running, or swimming. Flight requires more energy per unit of time, but because of the greater distance covered, it is also the cheapest mode of transport overall.

Most birds are of a size that enables them to become airborne and have wing shapes that ensure efficient flight. The wings are powered by massive breast muscles, the pectoralis and supracoracoideus, responsible for downward and upward strokes, respectively. 

The two pectoralis muscles, one on each side of the breast, are by far the largest muscle in the body of flying birds, forming more than one-third of the total body mass of some species. They are well supplied with blood vessels and consist of fast-contracting fibers (red fibers), which in many species can beat the wings continuously for hours or days on end.

Compared with other animals, birds are not only homoiothermic (warm-blooded), but they also have exceptionally efficient respiratory, cardiovascular, and metabolic systems. 

Unique features of birds used for migration:

Together these systems ensure that the specialized wing muscles are kept well supplied with oxygen, and energy-rich fuel, and that waste products are swiftly removed, preventing the muscle pain and fatigue so familiar to human athletes. 

The breathing mechanism of birds also results in much more efficient gas exchange than that in mammals. A bird’s lung is connected by an array of tubes to a system of thin-walled air sacs. Air is continuously directed through the lungs during both inspiration and expiration, thereby increasing the efficiency of oxygen extraction. 

Migration preparation:

By possessing all these various traits, birds are pre-adapted for the development of long-range movement patterns. Compared with resident bird species, migrants have these same features more highly developed as specialized adaptations for long-distance migration. It is this combination of features that enables some species of birds to perform some of the most remarkable migrations in the animal world.

Most birds migrate by flying, penguins and some other seabirds migrate by swimming, and some land-birds by walking for part or all of their journeys.

TYPES OF BIRD MOVEMENTS

The terms resident and sedentary are usually applied to birds that occupy the same general areas year-round and to populations that make no obvious large-scale movements resulting in changes in geographical distribution. The term migration is less easily defined because it means different things to different people. Ornithologists use the word only for return movements between breeding and non-breeding areas, but biologists working with other organisms often use the term more widely. For purposes of convenience in this book, I shall divide bird movements into six main types:

• First, there are the everyday routine movements centered on the place of residence, which occur in all birds, whether classed as resident or migratory. Typically, they include the flights from nesting or roosting sites to feeding sites or from one feeding site to another and can occur in any direction. In most birds, these movements are short and localized, restricted to a circumscribed home range, and extend over distances of meters or kilometers. But in other species (notably pelagic birds), regular foraging movements can extend over hundreds of kilometers out from the nesting colony.
• Second, there are one-way dispersal movements. In both sedentary and migratory bird species, after becoming independent of their parents, the young disperse in various directions from their natal sites. Individual young seem to have no specific inherent directional preferences, so within a population, dispersal movements seem to occur randomly in all directions. In most bird species, dispersal distances can be measured in meters, kilometers, or tens of kilometers, but in a few species (notably pelagic birds), such distances can be much greater. Post-fledging dispersal of this type does not usually involve a return journey, but in any case, most surviving young subsequently settle to breed at some distance from their hatch sites (called natal dispersal). In addition, some adults may change their nesting locations from year to year (breeding dispersal) or their non-breeding locations from year to year (here called non-breeding or wintering dispersal).
• Third, there is migration, in which individuals make regular return movements at about the same times each year, often to specific destinations. Compared with the above movements, migration usually involves a long journey over tens, hundreds, or thousands of kilometers in much more restricted and fixed directions. Most birds spend their annual non-breeding period at lower latitudes than their breeding period, but some migrate to similar latitudes in the opposite hemisphere where the seasons are reversed. Such migration occurs primarily in association with seasonal changes in food availability, resulting from the alternation of warm and cold seasons at high latitudes or of wet and dry seasons in the tropics. Overall, directional migration causes a massive movement of birds twice each year between regular breeding and wintering ranges and a general shift of populations from higher to lower latitudes for the non-breeding season.
• Fourth, there is another category of migration, which I have called dispersive migration, in which post-breeding movements can occur in any direction from the breeding site (like dispersal), but still, involve a return journey (like other migration). Although these movements occur seasonally between breeding and non-breeding areas, they do not necessarily involve any change in the latitudinal distribution of the population or any change in its center of gravity. They are evident in some land bird species usually regarded as ‘resident.’ They include altitudinal movements in which montane birds shift in various directions from higher to lower ground for the non-breeding season. In addition, many seabirds can disperse long distances in various directions from their nesting colonies over winter in distant areas rich in food, returning to the colonies the following spring.
• Fifth, there are irruptions (or invasion migrations), which are like other seasonal migrations, except that the proportions of birds that leave the breeding range, and the distances they travel, vary greatly from year to year (the directions are roughly the same but often more variable between individuals than in regular migration). Such movements are usually towards lower latitudes and occur in association with annual, as well as with seasonal, fluctuations in food supplies. In consequence, populations may concentrate in different parts of their non-breeding ranges in different years. Examples include some boreal finches that depend on the sporadic tree, seed, and crops and some owls that specialize in cyclic rodent populations.
• Sixth, there is nomadism, in which birds range from one area to another, residing for a time wherever food is temporarily plentiful and breeding if possible. The areas successively occupied may lie in various directions from one another. No one area is necessarily used every year, and some areas may be used only at intervals of several years but for months or years at a time, whenever conditions permit. The population may thus be concentrated in largely different areas in different years. This kind of movement occurs among some rodent-eating owls and raptors of tundra, boreal and arid regions and among many birds that live in desert regions, where infrequent and sporadic rainfall leads to local changes in habitats and food supplies. Because these changes are unpredictable from year to year, individual birds do not necessarily return to areas they have used previously and may breed in widely separated areas in different years.

Migration technique used by birds

These different kinds of movements intergrade, and all have variants, but in any bird population, one or two kinds usually prevail. 

Almost all bird species show post-fledging dispersal movements, in addition to any other types of movement shown at other times of the year. Some species show both nomadic and irruptive movements. Through migration, irruption, and nomadism, birds exploit the resources of mainly different regions at different times. The birds achieve greater survival and reproductive success (hence greater numbers) than if they remained permanently in the same place and adopted a sedentary (resident) lifestyle.

The main variables in these different types of bird movements include:

1. The directions or spread of directions.
2. The distances or spread of distances.
3. The calendar dates or spread of dates.
4. Whether or not they involve a return journey.

They also differ in whether they occur in direct response to prevailing conditions or an ‘anticipatory’ manner, in adaptation to conditions that can be expected to occur in the coming weeks, and leading birds to leave areas before their local survival would be compromised or arrive in other areas in time to breed when conditions there are suitable. Each of these aspects of bird behavior can be independently influenced by natural selection, given the great diversity of movement patterns found among birds related to the different circumstances in which birds live.

ADAPTATIONS FOR MIGRATION

One of the most amazing aspects of migration is how birds find their way over long distances. Many species are capable of migrating between the same breeding and wintering places year after year, even if these places lie thousands of kilometers apart on different continents. Young birds migrating alone can find their 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 particular nesting islands.

For example, Great Shearwaters Puffinus gravis nest on the isolated Tristan da Cunha islands. 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 45km of ocean. Individuals occupy the same nest burrows from year to year, often lying within a meter of other individuals. These and other seabirds that migrate long, overwater distances to small oceanic islands must surely be among the greatest animal navigators, possessing extremely accurate orientation mechanisms.

How birds find their way back

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 geo-magnetic 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. 

How do birds know its time for migration? 

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.

How birds utilize energy for migration

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. 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 in migratory birds are some of the most extreme in 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 their wings.

Migration needs and reaching destination in time

The migratory lifestyle requires that periods of movement are integrated with other events in the birds’ annual cycle, especially breeding or molt. In most bird species, these events normally occur at different times of the year, with minimal overlap. 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 molt and migration are separated as much as possible.

How do young birds know about migration? 

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 the 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 that promotes particular kinds of behavior at appropriate times of the year or stages in a journey. This inherent component of some bird movements adds fascination to the study of the controlling mechanisms.

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. 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.