Eight hornbill species occur in the Kinabatangan floodplain, a biodiversity hotspot in eastern Sabah, Malaysian Borneo. Through regular monitoring, Hutan, a community-based conservation NGO working in Kinabatangan since 1998, has detected a steady decline of hornbills in this area. We have determined that neither poaching nor lack of food is the primary cause of decline. Hornbills are cavity-nesting birds; the female and the chicks spend several months sealed in natural tree holes until fledging. Previous heavy timber extraction in Kinabatangan destroyed cavities suitable for nesting, and pioneer trees now colonize the regenerating forests. Tree cavities formed in these fast-growing trees have relatively short life spans, deteriorating as a result of heart rot. Their entrance closes rapidly, they are too small for large birds such as the Critically Endangered helmeted hornbill Rhinoplax vigil or the Vulnerable rhinoceros hornbill Buceros rhinoceros, and there is intense competition with other cavity users. The loss of suitable nest cavities directly threatens the long-term survival of hornbills in Kinabatangan.

In 2013, Hutan and several zoo partners (Beauval, Chester, Phoenix, Nashville, Woodland Park and Houston) built artificial nests for hornbills using plastic barrels. Hornbills soon visited the nest boxes, but as far as we are aware it was 3 years before one was first used, by a pair of rhinoceros hornbills. During 2017–2019, two rhinoceros hornbill pairs used these plastic nest boxes and produced five living chicks. The growth of lichen on the nest box surface not only created a more natural appearance but may have been instrumental in the absorption of light energy, reducing the temperature within the nest cavity. In total, we have installed 25 nest boxes using various models and materials (wood, fiberglass, plastic). Wooden boxes have attracted several pairs of Endangered wrinkled hornbills Aceros corrugatus, and bushy-crested Anorrhinus galeritus and oriental pied hornbills Anthracoceros albirostris used them for breeding. However, wooden boxes decay rapidly and are often colonized by stingless bees, civets, ants or flying squirrels, making them unsuitable for attracting large hornbill species. In 2020, no hornbills used the artificial or natural cavities during the breeding season (May–November) in Kinabatangan, possibly because rainfall was heavier than usual. Before the 2021 breeding season, we will erect more artificial nest boxes built with plastic drums, which last longer in harsh weather and tropical conditions. Although the occupancy rate of artificial nest boxes is less than 10%, this project gives us hope to sustain breeding populations of large hornbill species in degraded forests, which are becoming the norm across the range of most hornbills in Asia.