Birds have a very different reproductive pattern than mammals, better suited to the peculiar hazards of being a bird. The biggest problem with being a bird is that everyone is trying to eat you. Whereas most mammals (especially primates) have adopted a strategy of having relatively few offspring and devoting a fair amount of parental energy and time to caring for those few, most birds (with some notable exceptions) have gone the other way: they produce lots of offspring, give them some minimal amount of raising—in some cases, none—then toss them to the winds, literally and figuratively. The avian strategy is one of more or less unrestrained fecundity.
A nice example of this pattern is the mourning dove. Population studies over the decades have clearly shown that whether or not it is hunted (in some states these are protected as songbirds, in some they are considered game birds) about 70% of the chicks hatched every Spring are dead by the end of the year. They are prey to hawks, cats, crows and other birds, snakes, owls, squirrels, and a host of other predators. Most of them live only a few months. This doesn't even count the number of eggs that are smashed or dropped from the nest or never hatch. Nevertheless, come the next Summer, the dove population is back at its original level, thanks to their prolific production of young. The female bird's reproductive tract shows striking differences from the mammal's in consequence.
Avian reproduction has been best studied in the domestic chicken and turkeys, and the modern poultry industry rests on its complete understanding of the cycle and how to control it.
Birds lay eggs in clutches. A clutch consists of one or more eggs, followed by a rest period of about a day, then another egg or eggs. Clutch size is species-specific and is held within fairly tight limits. Pigeons (Columba livia) almost always lay clutches of two eggs. A good domestic hen will lay five or more eggs in a clutch, with an interval of a day between clutches. Clutch size, as well as the numbers of clutches laid in a breeding season, will vary with species, but the principle is the same.
In hens, ovulation usually occurs in the morning, and almost never after 3:00 PM under normal daylight conditions. The total time to form a new egg is about 25-26 hours. This includes about 3-1/2 hours to make the albumen coats (the "white"), 1-1/2 for the shell membranes, and about 20 hours for the shell itself.
Ovulation for the next egg of the clutch occurs within an hour of laying the previous one, and so the hen gets later and later in her timing each day; she "runs behind," like a clock that is improperly adjusted. Eventually she gets so far behind schedule that she would have to ovulate later than 3:00 PM. Since hens don't do that, the next ovulation is delayed, laying is interrupted; the break between clutches takes place and the cycle repeats itself a day or so later.
Hens (like codfish and mammals) are equipped with tens of thousands of potential eggs which theoretically could be laid, but (as with the oocytes of mammals) most of them never develop to the point of ovulation. A commercial laying hen might have as much as a year or even two of active life during which she produces nearly an egg a day. But when her production level drops off all the fun and games are over, and it's off to the Campbell's factory for a date with the noodle soup man. Sic transit gloria mundi!
The reproductive tract in birds differs significantly from that in mammals, and most of the parts, although they bear similar names to mammalian organs, have widely different anatomical, histological, and physiological features.
As with mammals, eggs are produced in ovaries, or, more correctly speaking, one ovary. In the majority of avian species only the left ovary is functional. Although the right one is present embryologically, it regresses during development and is vestigial in the adult bird.
The ovary is grossly divisible into two lobes, each of which contains follicles depending from stalks. Incidentally, the "yellow" of the egg you eat at breakfast is the true oocyte. The surrounding white parts are coats that are added later, as the egg passes down the tract. The yellow of the egg is a single cell and the yolk represents a colossal lipid inclusion in its cytoplasm! The largest single cell in existence is the yolk of an ostrich egg, which can easily be 6" in diameter.
The "follicles" in birds bear little resemblance to mammalian ones, histologically. There is no antrum, and no follicular fluid; instead the entire follicle is filled with a very yolky egg. The mass of yolk grows very rapidly. It provides all of the nourishment for the developing embryo, diminishing in size as the embryo grows. In chickens, post-hatching nutrition is also mostly from the yolk for a short time, as it's not completely absorbed when the chick hatches out.
To see the avian ovary click here.
Each follicle is surrounded by a well developed system of blood vessels. At the time of ovulation of a follicle, a whitish area, the stigma, appears on the surface of the follicle, and the egg is expelled into the upper end of the reproductive tract. Nothing resembling the mammalian corpus luteum is formed in birds.
These features can be made out on slides 221 and 680, and the variation in sizes among the follicles is a reflection of their different stages of development. Released eggs pass into the oviduct, also present only on the left side. The various divisions of the oviduct are all continuous with one another.
The expanded upper end is called, as in mammals, the infundibulum, seen on slides 207 and 702. The infundibulum has fimbriae, finger-like projections covered with cilia, as in mammals.
To see the fimbriae of the avian oviduct, click here.
But the infundibulum is more than just a funnel-shaped structure to catch the egg. It also makes the first of the overlying egg coats, the chalazae. These are the suspensory ligaments of the yolk. If you break an egg carefully into a bowl, you'll these as whitish stringy material on opposite poles of the egg. They serve to keep the embryonic disc properly oriented.
To see a longitudinal section of the infundibulum, click here.
The next portion of the tract is the magnum, seen on slide 699 (from the Latin for "large," because it's the longest portion of the system). The magnum produces the bulk of the egg white. The mucosa of the magnum is composed of simple epithelium which may be cuboidal to columnar (in this slide it's cuboidal). Some of the cells are ciliated and some are not.
There are enormous glands in the lamina propria/submucosa, which in fact hardly contains anything else. Under moderately high power, you'll see vesicles containing the protein secretions waiting to be released.
To view the magnum and its glands, click here.
Physical passage of the egg through the magnum is believed to stimulate the production and release of the albumen mechanically, and there is often enough to cover more than one egg. Hence the production of "double yolkers." This happens when two follicles mature and release their eggs at the same time. The eggs travel through the tract in tandem; if they're close enough together, they will be enveloped by the same albumen coat in the magnum (and later will be enclosed within one set of shell membranes and shell).
The third portion is the isthmus, seen on slide 700. The mucosal lining is intermittently ciliated simple columnar epithelium. Extensive glands in the lamina propria produce the soft shell membranes. These are the whitish ones which remain adherent to the shell when you break an egg into a bowl.
To see the avian isthmus, click here.
The next portion, the shell gland manufactures the calcareous shell in which the egg is laid. The demand for calcium to make the shell is very high, and so the circulating levels of blood calcium in birds are greatly elevated compared to mammals, typically twice as much. The epithelium of the uterus is pseudostratified, rather than simple, and is intermittently ciliated; the mucosa is formed into deep straight crypts with well vascularized CT supporting the cores.
To see the shell gland, click here.
The last portion of the tube is the vagina. This a muscular tube through which the egg is expelled to the outside world. The folds of the mucosa are short, and the lining is a tall columnar epithelium, again intermittently ciliated.
Near the junction of the vagina and the shell gland, there are deep glands lined with simple columnar epithelium. These are the sperm host glands, so called because they can store sperm for long periods of time (10 days to 2 weeks!). When an egg is laid, some of these sperm can be squeezed out of the glands into the lumen of the tract, whence they will migrate farther up to fertilize another egg. This is one of the really remarkable things about birds; the sperm remain viable at body temperature, something mammals can't do.
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