Approximately one-third of all bird species periodically move from wintering grounds to nesting grounds, i.e. make seasonal migrations. As a result, birds can live in areas with pronounced seasonal climate change, where cold, harsh winters replace the summer and have even mastered tundra areas, where they are provided with plenty of food in summer, but most of them cannot live in winter.
SEDENTARY AND MIGRATORY BIRDS
All birds can be divided into 3 large groups in an effort to change places. Sedentary birds spend all year round near nesting sites. Most of the settled birds are tropics, and pigeons and sparrows are among our birds. Nomadic birds make non-directional migrations for tens and hundreds of kilometers within the same natural zone where they nest (rooks, ticks, tits, owls). Migratory birds migrate to other natural zones, thousands of kilometers away from nesting sites (geese, cranes, starlings). The division is not very strict. For example, migratory ragweed thrushes in the years of rich harvest of mountain ash may not fly away for the winter. Birds of the same species living in the northern part of their range can be migratory and those living in the south can be nomadic or sedentary.
FLIGHT PREPARATION
The timing of the flight often depends on the way the birds are fed: grain-eating birds usually arrive earlier and fly away later than insect eaters. Birds have their own "internal calendar" that tells them when to fly away; the main example is the change in daylight hours. Signals about the approaching period of flight cause changes in the organism of birds. During migrations, birds travel considerable distances, and not all of them have the opportunity to stop and rest in the journey, especially if their route is over the ocean. For example, Pacific Golden Plover waders fly 4,000 km from Labrador to South America without stopping.
Therefore, birds are fed heavily before the flight, accumulating fat that can weigh half their body weight. In addition, some internal organs, such as the intestines, shrink, and the walls of the stomach are thinning (birds even belch up the stones they contain). Having undergone such "pre-flight training", the bird turns into a flying machine, which is far superior to any of the created by a human being in terms of economy.
Many puzzles of birds' navigational abilities have not been solved yet. It is not clear, for example, how some of them manage to find their exact destinations - islands lost in the oceans - or what guides young birds, who for the first time make their own long migrations. But scientists have managed to find out that birds flying in the daytime are guided by the sun, and they can take into account the change in its location. Species flying at night are guided by familiar constellations. It is also important to have an inborn knowledge of the duration of the flight, a kind of hereditary program. Most birds fly in packs, which makes it easier for them to fly.
BIRD RINGING
Ringing of birds helped to understand the migration routes of birds. Ornithologists catch birds with nets or traps and put aluminum rings on their feet with a number and address to which the person who got the ringed bird can send the ring. Mass ringing of birds on the scientific basis began at the turn of the XIX-XX centuries, now we can analyze the data on the movement of many millions of birds.
Many bird species fly from the mainland to the mainland, from the Northern Hemisphere to the Southern Hemisphere, crossing vast distances. Why don't they stop for the winter halfway around the equator? As it turned out, many migratory birds winter in places whose ecological conditions are similar to their nesting places, even though they are located in another hemisphere of the Earth. That's why polar terns nesting in the Arctic fly to the shores of Antarctica for wintering, flying a distance of 15-22 thousand km twice a year and spending about 90 days on this voyage.