At 12.5 to 14 cm (4.9 to 5.5 in) long, with a 29 cm (11 in) wingspan and a weight of about 18 to 23 g (0.63 to 0.81 oz), the Carolina Wren is a fairly large wren; the second largest in the United States species after the cactus wren. Among standard measurements, the wing chord is 5.4 to 6.4 cm (2.1 to 2.5 in), the tail is 4.5 to 5.6 cm (1.8 to 2.2 in), the culmen is 1.4 to 1.8 cm (0.55 to 0.71 in) and the tarsus is 2 to 2.3 cm (0.79 to 0.91 in). Sexual dimorphism is slight with males being larger than their mates. A study indicated that of 42 mated pairs, every male but one was larger than the female of the pair. The males were on average 11 percent heavier along with having longer wing chords.
Despite being socially monogamous, vermilion flycatchers will engage in extra-pair copulation. They also practice within-species brood parasitism, whereby females lay their eggs in the nest of another individual. Females build shallow open cup nests and incubate the brown-speckled whitish eggs. The male feeds the female during incubation. Two broods of two or three eggs are laid in a season lasting from March through June. Once hatched, both males and females feed the chicks, which are ready to fledge after 15 days.