jousting by Helmeted Hornbills: observations from Indonesia and Thailand
F. Kinnaird1, Yok-Yok Hadiprakarsa1 and Preeda Thiensongrusamee 2
1Wildlife Conservation Society-Indonesia Program, P.O. Box 311, Bogor,
Indonesia; 2 Thailand Hornbill Project, c/o Department of Microbiology,
Faculty of Science, Mahidol University, Rama 6 Road, Bangkok 10400, Thailand.
author: Dr M.F. Kinnaird. Email: [email protected]
Hornbills Rhinoplax vigil are unique among the Bucerotiformes in having
solid "ivory" casques that are used sometimes in aerial displays
where individuals collide casque-to-casque in mid-air (Kemp 1995). Knocking
and abrasion during these aerial combats may even help to shape the distinctive
flat, front profile of the adult birds' casques (Cranbrook & Kemp
1995). The behaviour has been reported only among males, although females
have bills and casques of similar structure and proportions to males (Sanft
1960, Kemp 1995). Interactions, which may last for up to 2 hrs, have been
attributed to agonistic interactions and territorial defense (Kemp 1995)
and to intoxication from consumption of fermented figs (Schneider 1945).
Published descriptions of aerial casque-butting by Helmeted Hornbills
are rare, and most observations are recounted tales of other observers
or, in the case of Kemp (1995) dramatized stories to illustrate the diverse
behaviour of the hornbill family (Cranbrook & Kemp 1995). In this
note, we describe six acts of aerial casque-butting, hereafter referred
to as aerial jousting, by Helmeted Hornbills. For all jousts, we recorded
the time of day, whether the jousting took place in or near fruiting trees,
and the number and sex of the 'actors'. We also report on the time and
location of five additional jousting events that were investigated only
after we were made aware of the behaviour by the distinctive clacking
sounds of colliding casques.
The majority of observations were made over a 2-month period (Sept 98
- Oct 98), as part of a larger study on hornbill foraging ecology (Hadiprakarsa
2000) at the Way Canguk Research Site, Bukit Barisan Selatan National
Park (BBS), Sumatra, Indonesia. Three additional records were made at
Way Canguk in 2000 and 2001, and one was noted at the Budo-Sungai Padi
National Park (BSP), Thailand in 2000. The Way Canguk Research Site covers
approximately 900 ha of disturbed and undisturbed lowland rainforest habitat
in the southeast portion of the 3,568 km2 BBS (Kinnaird & O'Brien
1998). BSP is a small forest isolate on a steep mountain outcrop (341
km2) near the border between Thailand and Malaysia and is similar to BBS
in being a mix of disturbed and undisturbed rainforest habitat (Uthai
Treesucon, Thailand Hornbill Project, unpubl. data).
Visual and auditory records were spread throughout the day, but the majority
occurred in the morning (Table 1). Jousting events generally involved
one male-male pair. Before jousting, perched males were observed hitting
tree branches with their bills and rubbing their bills from side-to-side
on the branches as though they were drilling for insect larvae or bill
cleaning. In two instances, males left their perches immediately after
hitting tree branches, flew in opposite directions, circled, glided towards
one another, and collided casque-to-casque in mid air, then returned to
their previous perch before repeating the aerial jousting. In BSP, Thailand,
however, males hit tree branches "like a boxer punching his gloves
at the corner of the ring before a fight begins", then called for
approximately 30 minutes before ascending into the air, jousting, and
returning to the same perch. When collisions occur, the resulting sound
(a loud "CLACK!") can be heard in the forest understory at least
100 m away. The collision, which generally occurs during a glide and not
while flapping the wings, can be so powerful that one or both birds are
thrown backwards, performing dramatic, acrobatic flips before righting
themselves and flying level (Fig. 1). Combatants were recorded approaching
one another at distances of 25-50 m.
On three occasions we observed more than one male-male pair jousting simultaneously
but we could not determine whether partners switched among the pairs or
jousted continuously with the same individuals. Jousting among more than
one pair of combatants took longer (up to 50 minutes) and usually involved
a greater number of actual jousting events (2-12 collisions) than single
pair interactions (1-4 collisions).
During at least two jousting events, females perched near the combating
males and were observed flying 1-2 meters above or to the side of their
males as they began jousting (Fig. 2a). When the males collided, the females
veered off in opposite directions and returned to their previous perches
to be joined immediately by their males (Fig. 2b). On one occasion, we
observed jousting between a male and female Helmeted Hornbill. The female
was perched in the upper canopy of a fruiting fig preening while the male
circled in a large loop overhead, then flew straight towards the female
and collided with her casque-to-casque. Immediately following, the male
perched next to the female and both preened their feathers.
Three of the six direct observations of jousting events occurred in or
near fruiting, hemiepiphytic fig trees (Ficus spp.). Of the remaining
three, one occurred near an active nest site between the breeding male
and a male intruder and the other two occurred near a large, leafless
tree snag. Three of the five auditory records also occurred near large
fig trees with ripe fruit crops. Therefore, seven of the eleven documented
jousting events were associated with defendable resources.
Although our data are not conclusive, we believe they indicate that aerial
jousting may have developed as an agonistic behaviour during resource
competition and not as a territorial display per se. Helmeted Hornbills
may compete for nesting sites and food resources, particularly fruiting
figs. Because Helmeted Hornbills prefer to nest in large trees (diameter
breast height: 105-216.6 cm) of the family Dipterocarpaceae, and choose
only those trees with distinctive knobs or stumps at the nest entrance
(Thiensongrusamee et al. 2001), nest sites are limited and competition
is likely. Helmeted Hornbills have been described as fig specialists (Kemp
1995) and studies in peninsular Malaysia and on Borneo and Sumatra confirm
the fruit portion of the diet to be 98% to 100% Ficus, with the animal
portion less than 1% in some areas (Hadiprakarsa 2000, Lambert 1989, Leighton
1982). Such heavy reliance on widely dispersed, low-density resources
with unpredictable, asynchronous fruiting patterns (Jansen 1979) would
seem to preclude year-round territoriality. Disappearance of Helmeted
Hornbills from the Way Canguk Research Site during certain months, and
congregations of more than eight pairs together in one tree, reinforce
the idea that these birds are not strictly territorial (Anggraini et al.
2000, O'Brien pers.com, Hadiprakarsa unpubl. data). The "territorial
calls" (Kemp 1995) of Helmeted Hornbills may function to coordinate
mated pairs that often forage far apart (Leighton 1982) or to announce
a pair's position to competitors whose home ranges may overlap, much like
the loud-calls of non-territorial Cercocebus primates (Waser 1976). Alternatively,
Anggraini et al. (2000) speculate that Helmeted Hornbills may practice
facultative territoriality, or variable resource defense, exhibiting territoriality
only when resources are limiting and defendable.
Our observations of male-female and male-male jousting in and near fruiting
figs support the idea that the behaviour may best be explained in the
context of competition for food resources. Simultaneous jousting among
several males also does not rule out resource defense but argues strongly
against territorial behaviour unless group defense is occurring. More
data are necessary to evaluate the function of aerial jousting among Helmeted
Hornbills; we hope that our ideas stimulate researchers to examine more
thoroughly the behavioural correlates of this little-known Asian hornbill.
We are grateful to Alan Kemp and Pilai Poonswad for encouraging us to
publish these observations, to Tim O'Brien, Alan Kemp and George Schaller
for helpful comments, and to Mohammad Iqbal, Risdianto and Wariyono for
assistance in the field. We thank our partners in Indonesia, the Directorate
for Forest Preservation and Nature Conservation, for permission to work
in Bukit Barisan Selatan National Park. The Wildlife Conservation Society
(MK &YK) and the Thailand Hornbill Project (PT) have kindly sponsored
and funded our hornbill studies over the years.
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