How do hornbills seal females inside nest cavities?

In most hornbill species, the female enters a nest cavity—typically located in a tree—before egg-laying begins. To seal the entrance, the female plasters the opening using mud and debris brought to her by the male. She continues this process until only a small slit remains.12

Through this narrow opening, the male provides food to the female throughout the incubation period and often while the young are growing. Once the young are sufficiently developed, the female breaks out of the nest. In some species, the young may then help to seal the nest again after the female emerges.12