They may offer the other bird seed, touching beaks briefly as they do so. During the season, males and females engage in courtship displays, swaying from side to side with necks outstretched, and crests erect, while singing softly. Northern cardinals are serially monogamous and pairs remain together for only one breeding season, rarely more. The legs and feet of almost all birds are thin and lack feathers, and are vulnerable to rapid heat loss. The down feathers are small and hairlike at the base of each flight feather. During winter, both males and females will fluff up their down feathers in order to retain warm air next to their bodies. Their beak and feathers (except for the black face mask) are yellow. A few "yellow morph" cardinals, a trait called xanthochroism, lack the enzyme to do this conversion. When fed only yellow pigments, males become a pale red color. Northern cardinal males normally metabolize carotenoid pigments to create plumage pigmentation of a color different from the ingested pigment. Coloration is produced from both red pigments and yellow carotenoid pigments. The plumage color of the males is produced from carotenoid pigments in the diet. They are brown above and red-brown below, with brick-colored crest, forehead, wings, and tail. Young birds, both male, and female show coloring similar to the adult female until the fall, when they molt and grow adult feathers. Both sexes possess prominent raised crests and bright coral-colored beaks. The face mask of the female is gray to black and is less defined than that of the male. The female is fawn-colored, with mostly grayish-brown tones and a slight reddish tint on the wings, crest, and tail feathers. The color becomes duller and darker on the back and wings. The adult male is a brilliant crimson-red color with a black face mask over the eyes, extending to the upper chest.
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