Bird migration mainly occurs in the Northern Hemisphere and is primarily driven by availability of food. They are funneled on to a specific route by natural barriers such as the Mediterranean Sea or the Caribbean Sea.
Migration is controlled primarily by changes in day length. Birds navigate using cues from the sun, stars, magnetic field and mental maps. The most common pattern involves flying north in the spring to breed and returning in the autumn to wintering grounds in warmer regions of the south. Within a population it is common for different ages and/or sexes to have different patterns or timing and distance: female chaffinches migrate earlier in the autumn than the males do (Panov, Illya N. 2011).
"Birds need to alter their metabolism in order to meet the demands of migration. The storage of energy through the accumulation of fat and the control of sleep in nocturnal migrants require special physiological adaptations. In addition, the feathers of a bird suffer from wear-and-tear and require to be molted. The timing of this molt - usually once a year but sometimes twice - varies with some species molting prior to moving to their winter grounds and others molting prior to returning to their breeding grounds" (Rohwer S : Butler LK 2005)." Apart from physiological adaptations, migration sometimes requires behavioral changes such as flying in flocks to reduce the energy used in migration "(Weber, Jean-Michel 2009).
"Most migrations begin with the birds starting off in a broad front. Often, this front narrows into one or more preferred routes termed flyways. These routes typically follow mountain ranges or coastlines, sometimes rivers, and may take advantage of updrafts and other wind patterns or avoid geographical barriers such as large stretches of open water. The specific routes may be genetically programmed or learned to varying degrees. The routes taken on forward and return migration are often different" (Newton, I 2008).
Birds fly at varying altitudes during migration, seabirds fly low over water but gain altitude when crossing land, and the reverse pattern is seen in landbirds. Bird migration is not limited to birds that fly however, most species of penguin migrate by swimming, and blue grouse and emus will migrate by walking.
Scientists use several techniques in studying migration, from banding to satellite tracking. One of the goals is to locate important stopover and wintering locations. Once identified, steps can be taken to protect and save these key locations.Resources: