The seal is built from droppings, food remains and mud, worked into a plaster that hardens over the cavity mouth. The female does the work from inside, narrowing the entrance to a vertical slit just wide enough to accept a bill tip. Once inside she is committed: she cannot leave until the wall is broken.
During confinement she undergoes a rapid moult, shedding flight and tail feathers, and depends entirely on deliveries from the male, who carries fruit in his gullet and regurgitates it item by item at the slit. The period runs to several months in this species. In Buceros hornbills the female typically breaks out before the chicks fledge, after which the young reseal the entrance and both parents feed them.
The arrangement makes the pair unusually fragile. Killing or displacing the male during confinement dooms the brood, and losing large cavity trees removes the nest sites entirely.