Secondary Sex Organs of Birds
The secondary sex organs of birds consist of the infundibulum, magnum, isthmus, uterus (shell gland), vagina and cloaca. The size of the entire reproductive tract is variable, depending on the species.
It is about 70-80 cm in fowl (Fig. 21.4) and about 60 cm in guinea fowl. The oviduct is divides into three parts. The first part of the avian oviduct is infundibulum (ostium). It captures the ovum (yolk of the egg) and is also considered the site of fertilisation in avian species. The next part of the oviduct is the magnum. It constitutes the
Fig. 21.4 Female reproductive system of birds. Figure shows the cluster of follicles with a mature ovum and with reproductive tract comprising infundibulum (10%), magnum or ampulla (50%) and isthmus, uterus and vagina, each containing 14% of the entire length of the tract. The position of the rectum, cloaca and rudiment of the right oviduct is also shown in the figure
Table 21.3 Time spent in the oviduct to form an egg in different domestic birds
| Part of oviduct | Fowl | Duck | Quail | Turkey | Guinea fowl |
| Infundibulum | 15-30 min | 15-30 min | 15-30 min | 15-30 min | 15-30 min |
| Magnum | 3h | 2 h 45 min | 2 h 15 min | 2 h 45 min | 2 h 30 min |
| Isthmus | 1 h 15 min | 2 h 15 min | 1 h 45 min | 1 h 15 min | 1 h |
| Uterus | 19-21 h | 18-19h | 19-20 h | 22-24 h | 20-21h |
| Vagina | Few min | Few min | Few min | Few min | Few min |
| Total | 24-25 h | 23-25 h | 24-25 h | 26-29 h | 24-25 h |
Data compiled from various sources
largest portion of the avian oviduct. The magnum has thick muscular walls containing an albumin secreting gland that helps form the albumin layer over the yolk.
The chalazae (in singular, chalaza) form during albumin secretion. Chalazae are the albuminous spring-like cords that originate from the two opposite ends of the yolk and extend towards the extremities of the egg to hold the yolk in position. The magnum proceeds to a narrow part of the tract called the isthmus. The soft shell membrane of the egg forms, and water and minerals add to the isthmus. The isthmus terminates at the uterus. It has secretory glands that form a hardy egg shell. The white colour of the egg is due to the secretion of calcium carbonate from the shell glands. Various colours or mosaic patterns of eggs in birds, lizards and reptiles are due to the secretion of different pigments from these glands that are controlled genetically to camouflage and protection from the predators. At the last phase of the shell formation, a gelatinous protein-rich fluid, called bloom, is formed over the shell. Bloom can protect the egg against the invasion of microorganisms. The next part is a narrow muscular walled vagina followed by a cloaca. The muscle of the vagina helps to expel the egg through the cloaca. Just before laying, the pores of the egg shell are sealed by the mucous secreted by the vaginal glands. The special vaginal tubules (spermatheca) are found in the vagina of birds, lizards and reptiles, which can store the spermatozoa. Spermatozoa are stored in storage glands (sperm nests) found in birds’ infundibulum and uterovaginal junction. Spermatozoa can be stored in the seminal receptacles located in the infundibulum in some snakes. An ovum takes at least 24 h to form an egg (Table 21.3). Hence, a bird can’t lay two eggs a day.21.9