Northern cardinals are socially monogamous, though polygyny occasionally occurs. Despite being socially monogamous, northern cardinals frequently engage in extra-pair copulations. In one study, 9 to 35% of nestlings were the result of extra-pair copulations.
Pair formation begins in early spring, and is initiated with a variety of physical displays. The male performs a variety of displays to attract a female, including courtship feeding. Breeding pairs may remain together year-round, and may breed together for several seasons. (Halkin and Linville, 1999)
Northern cardinals breed between March and September. They usually raise two broods a year, one beginning around March and the second in late May to July. The second nest is often parasitized by brown-headed cowbirds. Nests are built by the female in dense tangles of vines or twigs in shrubs and small trees. The female lays 1 to 5 (usually 3) white to greenish eggs that average about one inch in length and one-half inch in diameter. Incubation begins when the last egg is laid, and is performed solely by the female. The male brings food to the incubating female. The eggs hatch after 11 to 13 days of incubation. The female broods the chicks for the first 2 days. Both parents feed the chicks a diet of insects. Both parents also remove fecal sacs from the nest. The chicks begin leaving the nest 7 to 13 (usually 9 to 10) days after hatching. The parents continue to feed the chicks for 25 to 56 days after they fledge from the nest. After leaving or being driven out of their parents' territory, young birds often join flocks of other juveniles. They may begin breeding the next spring. (Halkin and Linville, 1999)
The female northern cardinal builds the nest, incubates the eggs for 11 to 13 days, and broods the altricial chicks for the first 2 days or so. During incubation, the male brings food to the incubating female. Both parents feed the nestlings a diet of insects and remove fecal sacs from the nest. The parents continue to feed the chicks for 25 to 56 days after they fledge from the nest. (Halkin and Linville, 1999)