That strange and mysterious phenomenon in the life of birds, their migratory journeys, repeated at fixed intervals, and with unerring exactness, has for thousands of years called forth the astonishment and admiration of mankind. (H. Gatke 1895.)
The most obvious feature of birds is that they can fly. This facility gives them great mobility and control over their movements. Many species can travel quickly and economically over long distances - up to thousands of kilometres, if necessary crossing seas, deserts or other inhospitable areas. They also have great orientation and navigational skills, and are able to remember and re-find remote places they have previously visited. Birds can thereby occupy widely separated areas at different seasons, returning repeatedly to the same localities from year to year, and adopting an itinerant lifestyle of a kind not open to less mobile creatures.
Although migration is evident in other animal groups, including insects, mammals, pelagic turtles and fish, in none is it as widely and well developed as in birds. The collective travel routes of birds span almost the entire planet. As a result of migration, bird distributions are continually changing - on regular seasonal patterns, and on local, regional or global scales. Movements are most marked in spring and autumn, but can occur in every month of the year in one part of the world or another. These facts raise questions about the ecological factors that underlie the movements and distributions of birds that simply do not arise with more sedentary organisms.
Birds are also pre-adapted for long-distance migration in ways that 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 time, but because of the greater distance covered, it is also the cheapest mode of transport overall. One type of flight, by soaring-gliding, is cheaper still, but is practised mainly by larger species, such as albatrosses, which can travel the Southern Ocean with little more energy expenditure than sitting still (Chapter 3). Long-distance flight also allows birds to cross hostile areas that would otherwise act as barriers to their movements. Nevertheless, while most birds migrate by flying, penguins and some other seabirds migrate by swimming, and some landbirds by walking for part or all of their journeys.
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 supra-coracoideus, which are responsible for downward and upward strokes, respectively. The two pectoralis muscles, one on each side of the breast, are by far the largest muscles 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 fibres (red fibres), which in many species can beat the wings continuously for hours or days on end.
Compared with other animals, birds are not only homiothermic (warm-blooded), but they also have exceptionally efficient respiratory, cardiovascular and metabolic systems. Together these systems ensure that the specialised 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. 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 specialised 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.
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