Male Helmeted Hornbill Buceros vigil.
Illustration by Daniel Giraud Elliott (1882).
The Helmeted Hornbill is both the largest and now the rarest of Borneo’s 8 hornbills due to poaching for the solid, orange ivory, casque that sits on top of the bill. The function of the casque is probably to act as a weight and shock absorber to the pointed bill which is used as a chisel when hunting for animals in tree holes. The casque also protects the skull during territorial fights. Both males and females have solid casques but only males take part in aerial jousting in which these hornbills clash head-on in mid air.
“Imagine being on the tropical island of Borneo and drifting quietly down a stream in a dug-out canoe The giant trees of the rainforest rise on each bank like cathedral spires, and the creepers which festoon them form cloisters that conceal the dark damp interior. Raindrops pattering on the foliage and the distant rumble of a retreating thunderstorm form a backdrop of sound, through which penetrates a single mournful hoot. More hoots follow at intervals, accelerating in tempo until they break suddenly into peals of maniacal laughter.
Two huge birds then burst across the dome of the sky, their naked red heads extended and metre-long tail feathers trailing behind. Cackling loudly, they ram into one another like mountain sheep. Their yellow casques, solid as ivory, clash with a loud crack before they return from their jousting to perch at pavilions in the forest canopy on each side of the stream. Male Great Helmeted Hornbills Buceros vigil are busy in defence of their territorial boundaries” Alan Kemp (1995) The Hornbills.
Helmeted Hornbill Diet: 50% animal prey and 50% figs
At Kutai in E. Kalimantan Leighton (1986) reported that Helmeted Hornbill pairs spent approximately 50% of their foraging time looking for animal food including lizards, birds and squirrels. The other 50% was spent feeding on figs. Helmeted Hornbills did not feed on fruit other than figs.
At Krau in Malaya, Frank Lambert (1989) found that Helmeted Hornbills only fed on 10 out of 38 fig species that fruited during his 3 year study period with a strong preference for the largest fruits of strangling figs in Section Conosycea including, F. delosyce, F. sundaica, F. pellucidopunctata, F.crassiramea, F.cucurbitina, F. dubia, F.stupenda, F. drupacea and F. subcordata. Large liana figs such as F. punctata and small Sycidium and Conosycea bird figs were ignored.
At Hala Bala in S. Thailand, Kitamura et al (2011) found that the 7 local hornbills fed on 93 different fruits including 10 species of figs. Helmeted Hornbills had the most specialised diet and only fed on 7 different fruits of which 4 were figs including F. benjamina, F. cucurbitina, F. dubia and F. sundaica.
At Bukit Barisan Selatan in Sumatra Hadiprakarsa and Kinnaird (2004) found that 4 different hornbill species fed on 64 different species of fruits of which 15 were figs. However Helmeted Hornbills fed exclusively on 12 different species of figs plus animal prey. These figs included F. albipila, F. crassiramea, F. drupacea, F. microcarpa, F. stupenda, F.sundaica and F. benjamina.
Helmeted Hornbill Territories
Helmeted Hornbills live in strictly teritorial monogamous pairs. Immature birds join nomadic flocks until they are ready to breed. Leighton (1986) reported that Helmeted Hornbills required the largest territory of any Bornean Hornbill before they could breed 7.7 km2 (770 ha). There is some dispute as to whether aerial jousting is related to territorial fights or the defence of fig trees, because jousting often takes place near fruiting figs. However these two concepts are synonymous. Due to the importance of large Conosycea figs in the diet the density of these fig trees defines the minimum size of the territory required for breeding. In forests with a high density of figs Helmeted Hornbills can survive in smaller territories. In forests with a low density of figs Helmeted Hornbills need larger territories.
At Bukit Barisan Selatan in Sumatra Hadiprakarsa and Kinnaird (2004) calculated a density of 1.9 Helmeted Hornbills per km2. The lowest density i.e. the largest territories of the 4 resident hornbills.
Density of Conosycea Strangler Figs in Virgin Lowland Rainforest
Harrison (2005), Lambert (1987) and Johns (1983) have all estimated the density of Conosycea stranglers at 2 to 3 individuals per ha. in virgin Sundaic rainforest, indicating that the minimum population of strangler figs needed to support a breeding pair of Helmeted Hornbills to be between 1,540 to 2,310 stranglers based on the Leighton’s (1986) estimate of a 770 ha. territory.
Trophic Cascades, Figs and Helmeted Hornbills at Lambir Hills, Sarawak
Rhett D. Harrison (2006) found 182 stranglers in a 60 ha Smithsonian tree plot (Density 3/ha) at Lambir in 1998. Eight years in 2005 later a replicate survey found found only 158 strangler figs (Density 2.6/ha. The reason was that mortality (57 individuals) exceeded recruitment (18 individuals). The most likely reason that mortality exceed recruitment is that between 1998 and 2005 nearly all large birds and mammals were hunted to extinction at Lambir.
Harrison’s figures show that the lifespan of a hemi-epiphytic stangler at Lambir is relatively short (25 years) due mainly to host tree falls. Thus the strangler population requires constant recruitment from the seed rain provided by dispersers for the fig population to be sustainable.
In summary a collapse in fig disperser populations leads relatively rapidly to a collapse in strangler populations. It is likely that even if poaching was controlled and Helmeted Hornbills were re-introduced to Lambir it would be too late. The density of figs would be too low for Helmeted Hornbills to survive.