Migratory birds’ physiology: A review

Migration has come from the latin word “migrare” may be defined as the seasonal movement of animals from one place to another. Bird migration was described by L. Thomson as “changes of habitat periodically recurring and alternating in direction, which tend to secure optimum environment conditions at all times”. Temperature, food supply, or the amount of daylight animals migrate, breeding is one of the purposes. Migratory birds undergo two migrations per year: a fall migration and a spring migration. Pre-migratory fattening is the behavior shown by the migratory birds in which they eat food in excess (hyperphagia) and gain weight quickly just to deal with the high energy demands for long migratory flight. This fueling is done before the migratory take off as well as at stopover sites of their routes. Fuel reserves reach maximum levels when birds start long non-stop flight. Feathers’ aerodynamic and insulatory functions are affected by long flight and its exposure to light therefore, the plumage is replaced periodically through a process known as molt. The corticosterone plays a major role in metabolic and behavioral functions to manage fuel utilization on a daily and seasonal basis. Migratory birds usually fly in a V formation and they follow a definite order during migration. Generally adults or old birds start first and the young follow them. Variety of senses helps in the navigation during migratory flight. It’s one of the peculiar system followed by the birds to know their route of migration and is followed by them each and every yearly of their flight. The most commonly used navigation tool is the use of sun compass. Using the sun for direction involves the need for making compensation based on the time. Other abilities include detecting the magnetic fields, visual landmarks and olfactory cues. Anatomical, physiological and molecular analyses have revealed that in birds independent clocks are present at a minimum of three levels: retina of the eyes, pineal gland and hypothalamus. Each of these has input, pacemaker and output components. It is becoming clearer that environmental input to the clock, such as seasonal changes in temperature, rainfall and food abundance, can regulate temporal relationships between physiology and behavior, linked with migration and reproduction.