Domesticated Muscovy duck
Both males and females reach sexual maturity at 1 year of age, even though they have not yet necessarily attained their full body mass. I've found Muscovy ducks to be willing and fertile breeders, regardless of whether both males and females reach sexual maturity at 1 year of age, even though they have not yet necessarily attained their full body mass.
I've found Muscovy ducks to be willing and fertile breeders, regardless of whether kept as single pairs or in a flock situation. The biggest problem with the flock-breeding approach is it's impossible to control inbreeding due to their polygamous nature. In a group setting I've observed that one or two of the males seem to be servicing all of the females, while the remaining males "hang out" in bachelor groups.
Of course, with such limited bloodlines available in North America, inbreeding, at least with cousins, has been inevitable. Unlike other species I haven't observed any obvious weakening, neurological problems or characteristic changes as a result nesting is greatly facilitated by the use of a variety of nest boxes, from large wood duck-like boxes with 6" to 8" diameter entrance holes to igloo shaped half barrels, to l' by 2' ground nest boxes with either side or end entry holes, to naturally occurring hollow trees and stumps.
The Muscovy hens are usually wonderful incubators, but frequently, at least in temperate climates, very poor broody mothers. It's purely a guess, but I suspect it's a result of their natural adaptation to a tropical climate where this skill is less critical. 
Muscovy ducks do not form stable pairs. They will mate in the water or on land, unusual for ducks, which normally mate on the water only. Domesticated Muscovy Ducks can breed up to three times each year. The Muscovy he lays a clutch of 8-16 white eggs, usually in a tree hole or hollow, which are incubated for 35 days. The sitting hen will leave the nest once a day from 20 minutes to one and one half hours, and will then defecate, drink water, eat and sometimes bathe.
Once the eggs begin to hatch it may take 24 hours for all the chicks to break through their shells. When feral chicks are born they usually stay with their mother for about 10–12 weeks. Their bodies cannot produce all the heat they need, especially in temperate regions, so they will stay close to the mother especially at night. Often, the drake will stay in close contact with the brood for several weeks. The male will walk with the young during their normal travels in search for food, providing protection. Anecdotal evidence from East Anglia, UK suggests that, in response to different environmental conditions, other adults assist in protecting chicks and providing warmth at night.
It has been suggested that this is in response to local efforts to cull the eggs, which has led to an atypical distribution of males and females as well as young and mature birds.
Male Muscovy Ducks have spiraled penises which can become erect to 20cm in one third of a second.
Females have cloacae that spiral in the opposite direction to try and limit forced copulation by males.  Although most male birds have no penis, ducks have a long corkscrew penis, and the females have a long corkscrew vagina, which spirals in the opposite direction. The males often try to force copulation, but the complex mating geometry allows the females to retain control—most of the forced copulations do not result in successful fertilization.
Courtship and Reproduction
Mating season starts in early spring at which time pairs of many species begin to engage in elaborate courtship displays. These displays involve movements and counter-movements that appear to be exaggerated preening, head bobbing, and feeding behaviors, as well as particular calls and postures—such as holding both head and tail up—that are unique to courtship. Sometimes these displays start in a social context that includes a large pool of potential mates.
Once paired, the male of some duck species will defend both a territory and the female from other males, while the female finds a suitable nest site and lays her clutch of eggs. Despite the sometimes fierce watch kept by the male, the female may mate with other males. Genetic analyses now available to scientists have revealed that this phenomenon occurs in many birds that were once considered strictly monogamous. After early incubation, the bond between the parents weakens, and the male of many species will abandon the female, leaving her to defend the eggs from nest predators such as skunks and foxes.
Ducklings imprint on their mother, a process that begins with the mother and ducklings exchanging low calls before the ducklings have even hatched. Almost immediately after hatching, most ducklings will follow the mothers into nearby water. The newly hatched ducklings are covered with a dense, insulating down that traps air, making them buoyant.
The young are able to forage immediately, but they still rely on the mother to defend them from predators. When necessary, she will call them to her with a "contact call." Maternal care in most species lasts until the young are nearly able to fly, but the duration of parental care varies across species. In Muscovy duck, the mother will watch over her brood for several weeks to several months.
The shape of an egg is an ovate spheroid with one end larger than the other end. The egg has cylindrical symmetry along the long axis.
An egg is surrounded by a thin, hard shell. Inside, the egg yolk is suspended in the egg white by one or two spiral bands of tissue called the chalazae (from the Greek word khalazi, meaning hailstone or hard lump.)
Air cell the larger end of the egg contains the air cell that forms when the contents of the egg cool down and contract after it is laid. 
Eggshell Egg shell color
This is caused by pigment deposition during egg formation in the oviduct and can vary according to species and breed
White (albumen) Egg Yoke the yoke in a newly laid egg is round and firm. As the yolk ages it absorbs water from the albumen, which increases its size and causes it to stretch and weaken the vitelline membrane (the clear casing enclosing the yolk). The resulting effect is a flattened and enlarged yolk shape.
Yolk color is dependent on the diet of the hen; if the diet contains yellow/orange plant pigments known as xanthophylls, then they are deposited in the yolk, coloring it. A colorless diet can produce an almost colorless yolk. Farmers may enhance yolk color with artificial pigments, or with natural supplements rich in lutein petals are a popular choice), but, in most locations, this activity is forbidden.
Some hens will rarely lay double-yoked eggs as the result of unsynchronized production cycles. Although heredity causes some hens to have a higher propensity to lay double-yoked eggs, these occur more frequently as occasional abnormalities in young hens beginning to lay. Usually a double-yoked egg will be longer and thinner than an ordinary single-yolk egg. Double-yoked eggs occur rarely, only leading to observed successful hatchlings under human intervention, as the unborn chickens would otherwise fight each other and die. 
It is also possible for a young hen to produce an egg with no yolk at all. Yolkless eggs are usually formed about a bit of tissue that is sloughed off the ovary or oviduct. This tissue stimulates the secreting glands of the oviduct, and a yolkless egg results. 
Avian Reproduction: Anatomy & the Bird Egg
Gonads - paired testes in males & usually a single ovary in females
Ovary Testes & follicles
Sexual organs increase dramatically in size as the breeding season approaches. In the bird's world the female birds can bias the sex of their chicks. Whether a bird is more likely to lay a male or female egg depends on which sex will have the greatest chance of doing well.
Well-fed females were more likely to produce daughters, while less well nourished birds were more likely to have sons. Female offspring need to be better nourished than males if they are to survive and grow well. In birds, it is the female’s egg rather than the male’s sperm that determines what sex the chick will be. Thus the female has the potential to determine the sex of her young by whether she ovulates a male or female egg.
All the resources given to the developing embryo are present in the egg at laying. Thus the size and the content of the egg are measures of the amount of resources that the female has allocated to that egg, which affects its subsequent survival chances. When females were fed on a low quality diet, they laid eggs that were considerably lighter than those laid when they were fed on a high quality diet, and they also laid far more male eggs on a low quality diet.
Incubation temperature and avian sex ratios
Although common in reptiles, incubation temperature has not been considered to be a factor in determining sex ratios. Sex -biased temperature-sensitive embryo mortality was greater at the lower and higher temperatures, and minimal at the middle temperature where the sex ratio was 1:1.
Copulation & fertilization
For most birds, copulation involves a 'cloacae kiss', with the male on the female's back & twisting his tail under the females competition. Time spent courting must be shown to predict sperm transfer or success to really back up the idea. The 1.5-cm appendage lacks blood vessels and has a twisted furrow down its length. Males in communal nests have longer ones than those that live alone, showing that size is a factor in social success. But for males at least, the phallus is for more than foreplay.
Occurs in seminiferous tubules of the testes Occurs best at slightly cooler temperatures, so spermatogenesis may occur primarily at night when body temperatures are slightly lower Sperm are stored at the terminal end of the vas deferens (seminal glomus), and this creates a swelling called the cloacae protuberance. Male birds have paired abdominal testes lying cranioventral to the first kidney lobe. Testes increase dramatically in size during the breeding season as in mammals, sperm formation is temperature sensitive, and maturation is assisted by nocturnal drops in temperature, or by the development of scrotal-like external thermoregulatory swellings holding the seminal glomera. In addition, male birds tend to have relatively low extragonadal sperm reserves and sperm are ejaculated soon after production in the testes.
Sperm competition and testes size
Comparative analyses suggest that a variety of ecological and behavioral factors contribute to the tremendous variability in extra pair mating among birds. In an analysis Pitcher et al. (2005) examined several ecological and behavioral factors in relation to testes size; an index of sperm competition and the extent of extra pair mating. In analyses, testes size was significantly larger in species that breed colonially than in species that breed solitarily, suggesting that higher breeding density is associated with greater sperm competition. In analyses testes size tended to increase with clutch size, which suggests that sperm depletion may play a role in the evolution of testes size. These results suggest that traditional ecological and behavioral variables, such as social mating system, breeding density and male parental care can account for a significant portion of the variation in sperm competition in birds.
Temperature and the timing of reproduction
Many bird species reproduce earlier in years with high spring temperatures, but little is known about the causal effect of temperature. Temperature may have a direct effect on timing of reproduction. As climate change has led to substantial shifts in timing, it is essential to understand this relationship to predict future impacts of climate change.
Most birds have only one ovary and one oviduct. In early stages of embryonic development, each female bird has two ovaries; only the left one develops into a functional organ. A mature ovary looks like a cluster of grapes. Which may contain up to 4,000 small ova which can develop into mature ova? With fertilization, the ovum (egg) becomes a developing embryo the embryo passes through the oviduct typically takes about 24 hours (for passerines & most other birds) The demand for calcium to make the egg shell is very high, and so the circulating levels of blood calcium in birds are greatly elevated compared to mammals, typically twice as much.
Ovary, oviduct, & egg with shell Source:
In most birds, only the left ovary and oviduct persist. The ovary enlarges greatly during the breeding season. Active ovaries resemble bunches of tiny grapes due to the developing follicles. The oviduct opens medially to it in a funnel-shaped ostium. Ovulation results in the release of an egg from a mature follicle on the surface of the ovary. The egg, with extensive food reserves in the form of concentric layers of yolk, is picked up by the ostium and ciliary currents
carry it into the magnum region. Over about three hours the egg receives a coating of albumen. The egg then passes into the isthmus, where the shell membranes are deposited. This takes about one hour. The egg them moves to the uterus, or shell gland, where the calcareous shell is added and, in some birds, pigment is added in characteristic patterns. The egg then passes into the vagina and cloacae for laying (Sotherland and Rahn 1987).
Males have an intermittent organ.
Explosive eversion and functional morphology of the duck penis
Co evolution of male and female genitalia in waterfowl has been hypothesized to occur through sexual conflict. The collagen matrix of the penis is very thin and not arranged in an axial-orthogonal array, resulting in a penis that is flexible when erect. To test the hypothesis that female genital novelties make intromission difficult during forced copulations, Brennan et al. (2010) investigated penile eversion. The results he found support the hypothesis that duck vaginal complexity functions to exclude the penis during forced copulations, and coevolved with the waterfowl penis via antagonistic sexual conflict. Near the junction of the vagina and shell gland of female birds are deep glands lined with simple columnar epithelium. These are the sperm storage tubules, so called because they can store sperm for long periods of time (10 days to 2 weeks). After an egg is laid, some of these sperm may move out of the tubules into the lumen of the tract, and then migrate farther up to fertilize another egg.