Birds are dioecious animals with pronounced sexual dimorphism (females differ from males).
Birds breed by laying eggs.
Males develop paired testes.
Females have one left ovary (eggs develop in it) and an oviduct.
Fertilization in birds is internal and occurs in the oviducts of the female.
The actual egg (ovum) is the yolk. All other formations are produced by the oviduct (a fertilized egg, moving along the oviduct, is covered with egg membranes – protein, fibrous and shell).
In the upper part of the yolk there is an embryonic disc (from which a bird embryo develops).
The yolk contains all the supply of nutrients and water necessary for the development of the embryo. The yolk is suspended on protein threads – chalases. This protects the embryo from jolts and always orientates the embryonic disc upward, which is important for incubating eggs.
Protein has a protective and nutritional function, providing the embryo with water. Outside, the squirrel is dressed in two thin parchment-like protein shells, which diverge somewhat at the blunt end of the egg and form an air chamber.
The shell performs the functions of protection and gas exchange. Gas exchange is possible only due to the porosity of the shell.
After the egg is laid, the process of embryo development begins in it.
The development of the embryo requires an elevated temperature (approximately 38 ° C). For this, the birds incubate their clutches of eggs (incubate chicks). The eggs are incubated by one of the parents or both alternately. The incubation of eggs in different species takes different times – from 11 to 85 days.
All birds take care of the offspring: they find food for them, warm them up, protect them from predators and teach them to fly.
Chick development types
Depending on the methods of feeding, protecting offspring and training chicks, birds are divided into brood and nesting.
Ducks, swans, chickens guard and train their chicks, which are born with open eyes and covered with down. Such chicks are called brood chicks. They are quite independent – they can follow their parents and find food.
In pigeons, swallows, swifts, sparrows and many other birds, chicks are born naked, blind, helpless. Parents feed them in their nests, therefore such chicks are called nesting ones.
Seasonal phenomena in the life of birds
There are two periods in the life of birds.
- nest construction;
- laying eggs;
- incubating eggs;
- feeding chicks;
- training chicks to fly.
- shedding and fat accumulation;
- gathering in flocks and moving over short distances in search of food;
- an increase in the duration of movements (migrations), their transformation into roaming or flights.
By the nature of migrations, birds are divided into sedentary, nomadic and migratory.
Sedentary birds do not fly far from their nesting sites. Their movements are associated with the search for food in nearby territories.
Wandering birds leave their nesting sites and often fly far away from them.
Migratory birds usually fly tens of thousands of kilometers away from their nesting sites to southern countries.
The main reason for bird migration is a decrease in food supplies.
The signal for the start of flights is a reduction in the length of daylight hours.