Multiplication – Just as the reproductive functions of the male and the female are different, so also their sexual organs differ accordingly. Most mammals and birds show marked sexual dimorphism. It manifests itself in differences in coloration (ducks) or in size (deer), by the presence or absence of antlers (jeleniowate), the shape of the paws (pheasants) or voice (deer, capercaillies). Reproductive cells (gamety) male sperm are produced in the testicles, and females - the eggs - in the ovaries. The reproductive cells mature during the mating season. In the female bird, there is one active ovary and fallopian tube - the right ovary is atrophic. The oviduct opens to the steak, from which the eggs are expelled to the outside. Female mammals have two ovaries, from which the eggs enter the uterus through the fallopian tubes. The uterus is connected to the vagina, the outer opening of which is under the anal opening. Mammals' copulatory organ is the penis. Male birds do not have a copulatory organ in most cases. The exceptions are drakes and gander. The sperm passes through the seminal cord and into the cloaca at the time of mating. Bird copulation occurs through the contact of the male and female cloacs.

In birds, the mating season occurs in spring or early summer. During this time, you can observe mating dances, accompanied by screams or other sound effects, birds assume different positions and perform dance movements of the body and acrobatic flights. Each species has its own distinctive mating dance, and the males often fight each other (grouse). Birds are often monogamous. Couples are formed for a period of one year, for several years or for life. Some species, like the capercaillie and the pheasant, they are polygamous; the male is surrounded by the entire mansion of females. Polyandry is a rare phenomenon, e.g.. the female woodcock is fertilized consecutively by many males. After the mating season, nesting occurs.

Birds build nests, in which the eggs hatch. Later, they are used to warm and raise the young. Sockets, of various shapes, typical of each species, they are built on the ground (partridge), on the tops of trees (grzywacz), in hollow or rotten trunks (the barn owl, owl, bruise pigeon), on the water or on the water's edge (duck). The number of eggs laid varies from species to species: the partridge carries fifteen of them, pigeon two. You can generally say, that species, whose young are breeders, having many enemies or, whose life is short, lay more eggs than others. The eggs laid in open nests are usually protective color (black grouse, kszyk), eggs of birds nesting in tree trunks (the barn owl, bruise pigeon) are white.

Some birds start brooding after the last egg has been laid (partridge, duck), and the chicks hatch simultaneously. Others brood after the female has started ascending, and the young hatch successively (predatory, the barn owl). It happens, that the firstborn pushes the last born young outside the nest or devours them. Females of some species (the barn owl, pheasants) they disembark on their own; in others, both parents sit down alternately (predatory, squab); in exceptional cases only the male broods (climber). The incubation period varies from species to species.

Nestlings are covered with fluff, they see well, they leave the nest early and can feed themselves (pheasants, ducks).

Nestlings are born naked, often blind. They must stay in the nest for a long time under the care of their parents (predatory, squab).

Most mammals are polygamous. After the mating season is over, males leave females, except for some species like wolves, which do not cease to live together. After conception, an embryo develops in the female's uterus. After they are born, the little ones start to suck their mother. The gestation period varies from species to species: in small game it is shorter, longer in large animals. Differences in the length of this period even occur in related species: pregnancy in a wild rabbit continues 28 days, at the hare 32. Weaselids and deer do not notice pregnancy for a long time.

Depending on the species, newborn babies are more or less developed. Little carnivores are born covered with hair, but they can't see. Young moths - on the contrary: they can see right after birth and are able to accompany the mother a few hours later. Generally, rodents and small carnivores have the most numerous offspring, and large mammals limited.