Coraciiformes TAG

Coraciiformes Taxon Advisory Group -

Knobbed Hornbill (Aceros cassidix)
SSP Manager: Eric Kowalczyk - Woodland Park Zoo

General Information

The Hornbill Taxonomic Advisory Group (TAG) has adopted taxonomy used by Kemp (1995). The older genus name Rhyticeros might be seen in the literature (Kemp 1979). Other common names in use include: Red-knobbed hornbill, Island hornbill, Buton hornbill and Celebes hornbill.


The adult male’s crown and back of head are reddish brown, face and neck are pale red or cream-colored, body and wings are black, and the tail is all white. A male’s bill is yellow with orange-brown ridges (“chevrons”) across the base of both mandibles, and features a tall red wrinkled ridge (also called the casque ridge) on the upper mandible near the eyes. The skin around the eyes is pale blue and the eyelids are dark blue. The bare throat skin is dark blue with a black band through the lower edge and turquoise skin below the band. The male has orange or red eyes, while legs and feet are black.

The adult female is smaller than the male, with similar coloring. The female’s casque is yellow, while the head and neck are all black. The female’s throat skin has a smaller black band and the eyes are brown or orange.

Both sexes of immature red-knobbed hornbills have plumage like the adult male when first emerging from the nest cavity. However, their casque ridge is undeveloped and the bill is pale yellow with a red wash at the base. Juveniles have pale facial skin and dark brown eyes with a yellow rim. At 10-13 months of age, the casque of juveniles begins to develop and the female begins to molt into adult head and neck colors (i.e. black). The juvenile male’s plumage remains the same with the casque growing larger and redder.


A. c. cassidix is restricted to the Indonesian islands of Sulawesi, Lembeh, Togian, Muna, and Butung where it occurs in evergreen forests up to 1800m. In South Sulawesi, this species has been declared the state bird. Kinnaird and O'Brien (1996) have documented high densities around fruiting trees but caution that the species may be declining as forest habitat is also declining. It is difficult to accurately census a nomadic species that congregates around fruiting trees; counts based on temporary, mobile aggregations may result in an exaggerated estimate of the population's true numbers. A second subspecies, A. c. brevirostris occurs on the islands of Butung and Muna. Kemp (1995) suggests that this form represents the southern end of a cline through Sulawesi.


Red-knobbed hornbills inhabit primary lowland forest and forest edge from sea level to about 3,600 feet (1,097 m) in elevation. They occasionally range up to 5,900 feet (1,798 m) in elevation. Radio-telemetry studies have shown that red-knobbed hornbills may cover as much as 23 miles (37 km) per day, passing through degraded habitat in search of fruiting trees.


Red-knobbed hornbills are not an endangered species and are common throughout their range. Other species of hornbills are not so fortunate, as at least four species are listed as endangered.

The Sulawesi red-knobbed hornbill is endemic to the island of Sulawesi, which is slightly larger than the state of North Dakota and encompasses just 73,057 square miles (189, 216 km2). Therefore, the red-knobbed hornbill’s habitat is restricted and the small size of Sulawesi limits its future survival. Today, the biggest threat to this hornbill is logging, which results in loss of habitat. Excessive logging removes the required large trees with naturally occurring cavities. To fully assess the status of this species, researchers need more studies to determine if the population is stable or declining.

Hornbills are important because they help disperse seeds throughout the forest. They regurgitate larger seeds, and pass smaller seeds through their feces. It may be that forests need hornbills as much as hornbills need forests. The presence of nesting hornbills is a sign of a healthy Asian tropical rain fore.




Sulawesi red-knobbed hornbills probably reach sexual maturity at 5-6 years. In Sulawesi, the breeding season normally occurs from June to September. If the first clutch fails, the birds will re-nest and try again in July or August. Most North American captive births result in hatchings between April and June.

The birds find a natural cavity (caused by lightning strikes, natural decay, etc.) in a large tree and the female seals herself inside. She creates a mixture of feces, food, and feathers into a clay-like substance to seal off the enclosure. A narrow vertical slit in this hard wall provides space for the female to defecate and the male to regurgitate food to the female (and nestlings). She remains inside the nest, incubating two to three white eggs for approximately 30-35 days. During incubation, the female depends entirely on the male to provide her with food. After hatching, the female remains in the nest, brooding the chicks for approximately another 90 days.

In the wild, the chicks emerge from the nest some 30-40 days after the adult female has exited the nest cavity. It is rare that more than one chick successfully fledges. After confinement inside the cramped nest for as much as 130 days, the fledgling is not a strong flyer. It takes a day or two of exercising its muscles to enable the fledgling to keep up with its parents.Parents continue to feed the juvenile as they forage together in the forests.

Video of a knobbed hornbill perched in a tree can be accessed by clicking here. Video of the hornbill at the nesting site can be accessed by clicking here.

Current Population

The first documented captive breeding in this region occurred at Audubon Park and Zoological Garden in 1991. Michi (1993) wrote about breeding this species at Walsrode Vogelpark at Mallorca, Spain.

Recent successful reproduction occurring in this region are from pairs at St. Augustine Alligator Farm (Krueger 2010) and San Diego Wild Animal Park. Reproduction at AUDUBON represents the first time a F1 bird has bred. The sire hatched at SEATTLE in 1996. Although the chick did not survive, this is encouraging for developing a self-sustaining captive population. Reproduction at St. Augustine in 2009 marked the first successful reproduction from two F1 birds. (#70, 55). Unfortunately with the loss of several wild caught birds in 2010, this North American captive population is at risk of going endangered in the near future.

This studbook includes 80 historical records. As of early 2011 there are 6.8.1 living birds in 8 institutions recorded in this studbook


In the wild various fruits (especially figs) are the major part of their diet. Red-knobbed hornbills also eat small invertebrates as a small part of their diet.

(Aceros cassidix) Gallery