Bird Migration© Mazhar Ali
Apr 7, 2000
Some birds, such as house sparrows, remains at a particular place or may shift to a nearby place, are known as permanent residents. Included in this category are also such birds that make a daily or routine flight from one place to another. But many species migrate to or shift regularly from one region to another with the change of season. These birds come back not only to the same region but also to the same place and sometimes to place in search of food and do not make seasonal migrations. Such birds are known as birds of passage. Most migrations are latitudinal or north and south. Birds move into the wide land masses of the north temperate and subarctic region where there are facilities for feeding and nesting during the warmer months, and then come back to south for the winter. But there are similar movements also in the Southern Hemisphere where the seasons are reversed. This is in lesser extent. Some birds perform latitudinal migrations into mountainous regions for the summer and return to the low lands for the winter. Some tropical birds migrate to breed in the rainy season in the outer tropics, and come to central tropics in the dry season. Marine birds also make extensive migrations. The great shearwater comes as far as Greenland, returning to sea in the lower regions. Penguins make migrations for swimming.
The main reason for migration is supposed to be the scarcity of food and nesting. Insectivorous and fruit-eating birds find their supply cut off with the change of season, so they have to go in search of food to new places. But birds are also supposed to have a stimulus to migration, particularly those having north-south migrations, and there is experiment evidence to prove it. The stimulus is provided by the internal conditions of the gonads influenced by the hormone due to the seasonal change with particular reference to the shortening or lengthening of the days.
Most species have established routes to follow and travel more or less on schedule, arriving and disappearing regularly as if they had a calendar. Birds certainly do not learn the routes from their elders, because new generations may follow the particular route on their own accord without being accompanied by their elders. It is believed that this ability to follow the same course may depend on the ability to navigate by observation of the position of the sun or stars. The phenomenon that older ones may return to their old nesting sites and the younger to the approximity of their birthplace reveals that it may be due to their sharp memory and sharp sight. It is just possible that they may be guided by instinct impressed on the nervous system in some way through countless generations. Some migrants follow obvious landmarks such as coastlines, mountain ranges and rivers that help them in directional features. Sometimes young may follow their parents in the first journey, but the whole phenomenon is still not very clear. The coming back of the pigeon to a particular place even if released at a distance as far as five hundred miles is definitely believed to be due to the capacity to navigate by sun's altitude, horizon and movement. Shearwater was found to return home from a distance of about three thousand miles in twelve days.
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