American white ibises nest in mixed colonies (with other ibis species and herons). The amount of available food determine when pairs form. This means that the breeding season begins at different times from one year to the next.
Courtship behaviour begins with the male and female smoothing each other's feathers with their bills and then with a twig held in their bills. The nest, a platform of twigs and reeds, is built from May to July. The male brings the material to the female, who builds the nest.
A colony starts to form with a flock of males, and then the females arrive and build their nests nearby, in a live or dead tree.
Both the male and female incubate the two, three or four white or cream-coloured eggs, which take 21-23 days to hatch. The young are blind when they are born and remain lying down in the nest for their first week, with the parents protecting them from the sun's rays under their outspread wings.
The young leave the nest at an age of 28 to 35 days, but they are fed by their parents until 40 to 50 days after they’ve hatched, at which point they will leave the colony. They can reproduce at the age of three years.