Male and female ruddy ducks exhibit distinct plumage differences, particularly during the breeding season. Breeding male ruddy ducks have a striking reddish-brown body, a black cap, and bright white cheeks, contrasted by a sky-blue bill. Females and non-breeding males are a duller grayish-brown overall, with a dark line across their pale cheek patch. They also have a dark gray bill.

Male (Breeding Plumage):
• Body: Rich chestnut or reddish-brown.
• Head: Black cap, contrasting with bright white cheeks.
• Bill: Bright, powder blue or sky-blue.
• Other: Often holds its long, stiff tail cocked upward.

Female and Non-Breeding Male:
• Body: Dull grayish-brown overall.
• Head: Dark cap, extending below the eye, with a pale cheek patch.
• Cheek: A dark line or stripe across the pale cheek patch.
• Bill: Dark gray.
• Note: Non-breeding males resemble the females, but maintain their white cheek patch.