Why do birds fly south in winter?

Lan ying

Bird migration's reasons are very complicated. It is generally believed that this is an instinct of birds. This instinct is not only the result of genetic and physiological factors, but also the result of long-term adaptation to external living conditions, which is closely related to changes in living conditions such as climate and food. Migratory birds are very sensitive to climate change. As soon as the climate changes, they begin to migrate. In this way, we can avoid the cold in winter in the north and the heat in summer in the south. Climate change also directly affects the food conditions of birds. For example, after autumn, most plants in northern China withered their leaves, and insect activities decreased. After laying eggs, they successively burrowed into the ground to sting or die, and the number dropped sharply. The lack of food makes it impossible for birds that eat insects to sustain their lives. Only by migrating to the food-rich south can they survive the winter well, and insect-eating birds and small insect-eating birds also move south.

Weather, wind direction and wind force have great influence on bird migration. It is more suitable for sunny days with downwind of 3-5 winds. However, some birds migrating in spring sometimes rush to reach the breeding ground because the breeding season is approaching, so they will overcome difficulties and continue to migrate even under very unfavorable climatic conditions.

What's even more amazing is that birds can fly very harmoniously during migration, sometimes turning to the left, and sometimes jumping in the air like horses, which is very spectacular. Since ancient Roman naturalist Pirini recorded the observation of geese and other birds for the first time, people have studied and explored this phenomenon for 20 centuries, but there are still different opinions. At present, there are three explanations: one is the theory of "energy saving", according to the principle of "aerodynamics" or "runway", when birds fly in a "V" shape, they can save about 70% energy by putting their wings on the airflow generated by other birds, which is very important for geese with heavy bodies; The second is "information", which means that when birds fly, there are often one or several experienced leading birds leading the way, and the leading birds can provide reliable information on food sources and water sources for the birds. The third is the "safety" theory, which holds that when a large group of birds gather together, it is easier to find the enemy's harm than when there is only one bird or only a few birds, because when birds fly or inhabit, as long as one of them finds the enemy's harm, it will quickly transmit this information to all the birds in the form of one bird spreading to ten, and the birds will immediately take emergency measures, or run away quickly, or sing together to scare away the enemy's harm.

Many birds have an instinct, the so-called "homing nature", which reflects their attachment to the birthplace of Zaoxiang and their ability to find their hometown. It can help birds return to their nests smoothly in the second breeding season. Someone once caught a vulture owl. 13 years later, this free bird returned to the place less than 2 kilometers away from the old address. Birds can find their way from thousands of miles away, which has always been one of the mysteries of magical nature. What do they rely on to decide their own course? Polaris? The sun? The moon? Wind? Climate? Or geomagnetism? Where does their sense of direction come from? This has always been a mystery in nature. Scientists have found that birds often rely mainly on vision, with the help of ring marks, radar, flight tracking and remote sensing technology, and determine the flight direction through the positions of the sun, moon and stars in the sky. In addition, topography, rivers, thunderstorms, magnetic fields, polarized light, ultraviolet rays, etc. Are the basis for birds to fly thousands of miles without getting lost. Recent studies also show that there are nerve cells in the cortex of bird beak that can distinguish magnetic fields, and nerve cells called pineal gland play an important role just like vertebrates' sensory organs for light. Many electrophysiological experiments on mammals and homing pigeons show that some pineal cells can respond to small changes in magnetic field strength.

It is generally believed that birds that migrate during the day are located according to the sun, and birds that migrate at night are located according to the starry sky. Another view is that birds have keen eyesight to adapt to aerial observation. In the open environment, the visual radius of human beings is 9.6 kilometers, while the vision of birds flying at an altitude of 2000 meters is 100 kilometers. They can also remember the familiar features of vast areas as directional signs, which plays a key role in their migration from breeding grounds to wintering grounds.

The migration of birds is by no means easy. Usually, after flying over the vast sea and tall mountains, its weight will be reduced by half, and a large number of young birds born in that year can't escape the fate of dying during the migration or after reaching the migration terminal. During the migration, there is no time to feed, sudden storms, vast waters, etc. They devour these creatures all the time. At the same time, there is also a crisis in the migration time. Too early means that the living environment in the north is still covered with snow and ice, too late will be in danger of storms, and there will be countless human disturbances: tall buildings, radio antennas, lighthouses and chimneys, collisions with planes and so on. , are lurking in the long bird migration.