Unlike fish and amphibians, in birds, as well as in reptiles, the eggs mature and meet the sperm produced by the males inside the female’s body (what we call internal fertilization). The reproduction of birds begins with the courtship (or procession) usually composed of dancing or singing from the male to the female, but remember that in some exceptions, the female is the one who does all this staging!
After winning the partner, the male climbs on the female, superimposes his cloaca (genital opening) with hers and transfers his semen in which are the sperm, which follow the path through the female oviduct until they find the eggs and fertilize them.
All birds are oviparous (they lay eggs, into which the embryo develops, with no connection to the mother’s body), and in addition to practicing internal fertilization, they perform parental care (one or both parents take care of the young, either by hatching the eggs , as they need to be warm for the embryo to develop, or to feed them after birth).
Some birds build their own nests, others use the nest of other birds to lay their eggs, as is the case of the opportunistic bird (or parasitic bird), the Chupim (also known as Carixo, Papa-rice …). This bird takes advantage when the bird that owns the nest is not present and lays its egg among hers, without realizing it it ends up “adopting” Chupim chicks, which are usually born first and eat all the available food, usually causing the death of others puppies.
Some species of penguins, for example, are monogamous (choose a partner and make a couple “forever”), while other birds have several partners or even change partners every year. One study suggests that the “fidelity” found in some penguin species, can be explained by the fact that they meet infrequently (once a year for copulation).
Sexual dimorphism in birds
Sexual dimorphism, physical (also called morphological) non-sexual characteristics present in females and males, which make it possible to differentiate the sexes, is quite characteristic in birds. When it exists we can easily differentiate the male (usually more colorful) from the female, but it is worth remembering that not all species of birds have sexual dimorphism and that in some species the dimorphism is the opposite, with the female being the most colorful.
The sexual identification of birds is not always easy, chicks rarely have characteristics that can help sexual differentiation, and about half of the birds in the world do not have sexual dimorphism as adults. Sexual separation can then be done by chromosomal identification. While in mammals, females are the homogametic sex, presenting the pair of chromosomes XX and the males, the pair XY, in birds, males are homogametic (ZZ) and females heterogametic (ZW). By sequencing the DNA of most birds, one can identify a specific gene that is linked to the W chromosome and another that is present on the Z chromosome, so we can genetically identify the sex of the birds.
As for the reproductive organs of birds, females have two ovaries, two oviducts (divided into infundibulum, magnum, isthmus, uterus and vagina), however both the oviduct and the right ovary are atrophied and the left increases in size only during the reproductive phase. . These characteristics were selected during the evolution, contributing to the flight.
The left ovary has both cellular (oocyte production) and endocrine (hormone production) functions, its size varies according to its functional state and its color is usually yellow. It is known that the hormones estrogen, progesterone and androgens are important for the development and functioning of the reproductive system of birds.
During ovogenesis (it is at this stage that what we know as egg yolk is formed), the ovary produces and releases mature follicles (also known as oocytes), which are captured by the infundibulum and in it, if there is copulation, fertilization occurs. (it is normal to enter more than one sperm, but only one fertilizes each egg and the others degenerate).
After the encounter of the female and male gametes, the zygote is formed, embryogenesis begins (formation of the embryo) and during the passage of the egg through the oviduct, it receives different important components for its formation. In the oviduct, the magnum (or also known as the albuminiferous gland) produces the albumen (egg white), the isthmus is the place where the formation of the membranes of the egg shell occurs, and the uterus (also known as the shell or chamber gland). calciferous) is the place where the shell finishes being formed, it is there that the egg receives different components such as: calcium carbonate, cuticle, pigments (if any), proteins.
After passing through the uterus, the egg is sent to the vagina, where it will receive a layer of mucus and finally sent to the cloaca (the common opening between the intestinal canal, urinary tract and genital tract of birds). About a day after ovulation, the egg is already formed and ready for laying.
Males have two testicles, two vas deferens and cloaca. In most species of birds, males do not have a copulatory organ, but a small portion of species has a rudimentary phallic body, in which the size can vary from 1-3 mm (in cocks) and 5 cm (in ducks). In the shape of beans, young people have yellowish testicles while adults have white testicles, with the left testicle being larger than the right. As it is located inside the body of birds, it works at body temperature, different from what occurs in mammals, in which thermoregulation occurs.
The testicles are coated by the tunica albuginea and have a dual function, producing gametes and hormones, such as testosterone, androgens and estrogen. Testosterone is important in the growth and maintenance of sexual organs and in cutting behavior, while androgens boost the development of ridges, spurs and dewlap.
Connected to the testicles, the vas deferens end in the cloaca, the epididymis is a very short organ which has no importance for the maturation of sperm. As in birds there are no bulb-urethral glands, prostate or vesicular glands, the volume of semen is very small.
Environmental influence and reproduction of birds
There is an environmental factor that directly influences the reproductive activity of birds, photoperiodism. The bird has a reproductive period according to the “optimal period” for the survival of its offspring, depending on the length of the day, sexual activity increases or decreases. In commercial poultry breeding, it is common to use both the presence of light to initiate the reproductive activity of birds, and the suspension of it to delay the activity of the gonads.