Angry Muscovy duck.

All Muscovy ducks have long claws on their feet and a wide, flat tail. In the domestic drake (male), length is about 86 cm (34 in) and weight is 4.6–6.8 kg (10–15 lb), while the domestic hen (female) is much smaller, at 64 cm (25 in) in length and 2.7–3.6 kg (6.0–7.9 lb) in weight. Large domesticated males often weigh up to 8 kg (18 lb), and large domesticated females up to 5 kg (11 lb).

The true wild Muscovy duck, from which all domestic Muscovies originated, is blackish, with large white wing patches. Length can range from 66 to 84 cm (26 to 33 in), wingspan from 137 to 152 cm (54 to 60 in) and weight from 1.1 to 4.1 kg (2.4 to 9.0 lb). On the head, the wild male has a short crest on the nape. The bill is black with a speckling of pale pink. A blackish or dark red knob can be seen at the bill base, which is similar in colour to the bare skin of the face. The eyes are yellowish-brown. The legs and webbed feet are blackish. The wild female is similar in plumage, but much smaller, with a feathered face and lacking the prominent knob. The juvenile is duller overall, with little or no white on the upperwing.

Domesticated birds may look similar; most are dark brown or black mixed with white, particularly on the head. Other colors, such as lavender or all-white, are also seen. Both sexes have a nude black-and-red or all-red face; the drake also has pronounced caruncles at the base of the bill and a low erectile crest of feathers.[8] C. moschata ducklings are mostly yellow with buff-brown markings on the tail and wings. For a while after hatching, juveniles lack the distinctive wattles associated with adult individuals, and resemble the offspring of various other ducks, such as mallards. Some domesticated ducklings have a dark head and blue eyes, others a light brown crown and dark markings on their nape. They are agile and speedy precocial birds.

The drake has a low breathy call, and the hen a quiet trilling coo.

The karyotype of the Muscovy duck is 2n=80, consisting of three pairs of macrochromosomes, 36 pairs of microchromosomes, and a pair of sex chromosomes. The two largest macrochromosome pairs are submetacentric, while all other chromosomes are acrocentric or probably telocentric for the smallest microchromosomes. The submetacentric chromosomes and the Z (female) chromosome show rather little constitutive heterochromatin (C bands), while the W chromosomes are at least two-thirds heterochromatin.

Male Muscovy ducks have helical penises that become erect to 19 cm (7 in) in 0.3 s. Females have vaginas that coil in the opposite direction that appear to have evolved to limit forced copulation by males.