Black-crowned night herons are presumed to be monogamous. Pair formations are signaled by males becoming aggressive and performing Snap Displays, in which they walk around in a crouched position, head lowered, snapping their mandibles together or grasping a twig. The Snap Display is followed by the Advertisement Display--sometimes called the Stretch, Snap-hiss, or Song and Dance display--to attract females. In this display a male stretches his neck out and bobs his head, and when his head is level with his feet, he gives a snap-hiss vocalization. Twig-shaking and preening may be occur between songs. It has been suggested that these displays provide social stimulus to other birds, prompting them to display. This stimultion in colonial species may be crucial for successful reproduction. Females that come near the displaying male are rejected at first, but eventually a female is allowed to enter his territory. The newly-formed pair then allopreens and engages in mutual billing. At the time of pair formation, the legs of both sexes turn pink. Copulation usually takes place on or near the nest, and begins the first or second day after the pair is formed.
There is one brood per season. Black-crowned night herons nest colonially, and often there can be more than a dozen nests in one tree. The nest is built near the trunk of a tree or in the fork of branches, either in the open or deep in foliage. The male initiates nest building by beginning to build a new nest or refurbishing an old one. The nest is usually a platform lined with roots and grass. During and after pair formation, the male collects sticks and presents them to the female, who works them into the nest. The male's twig ceremony gradually changes to nest building.
The eggs are laid at 2 day intervals, beginning 4-5 days after pair formation. Incubation, which lasts 24-26 days, is carried out by both adults. The clutch size is 3-5 eggs. The eggs are greenest on the first day and fade to pale blue or green after that. On hot days, the parents wet their feathers, perhaps to keep the eggs cool. Both parents brood the young. After 2 weeks, the young leave the nest, although they don't go far. By 3 weeks, they can be found clustered at the tops of trees if they are disturbed. By Week 6-7 they fly well and depart for the feeding grounds. Adult black-crowned night herons do not recognize their own young and will accept and brood young from other nests. The young have a tendancy to regurgitate their food onto intruders when disturbed. (Davis, 1993)
Average time to hatching: 25 days.
Average eggs per season: 4.
Average age at sexual or reproductive maturity (male)
Sex: male: 730 days.
Average age at sexual or reproductive maturity (female)
Sex: female: 730 days.