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Aerial casque-butting in the Great Hornbill
Buceros bicornis

By T. R. Shankar Raman
 
Note: This article was originally published in Forktail 13 (1998) the journal of the Oriental Bird Club (OBC) and was kindly submitted by T. R. Shankar Raman

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Recently, Cranbrook and Kemp (1995) drew attention to the phenomenon of aggressive interactions among Asian hornbills (Bucerotidae) involving individuals (males) clashing their casques in mid-air flight. Among the six genera and 31 species of Asian hornbills now recognized (Poonswad and Kemp 1993, Kemp 1995), such aerial casque-butting has been reliably reported only in a single species of large hornbill, belonging to the genus Buceros. This is the Helmeted Hornbill, Buceros (subgenus Rhinoplax) vigil, which was only recently placed in this genus (Kemp 1955). A reference to the existence of aerial casque-butting behaviour in the Great (Pied) Hornbill, B. bicomis, was made in Poonswad and Kemp (1993, p. 104); this was, however, later reported to be an error (Cranbrook and Kemp 1995). All species of Buceros are territorial as adults when breeding, and it is of much interest, particularly in the face of cladistic changes in the taxonomy, to see which aspects of behaviour are shared among the species.

Here, I report field observation of the rare aerial casque-butting behaviour in the Great Hornbill. During a sixmonth study of the impact of shifting cultivation on tropical rainforest bird communities (Raman 1995), aerial casquebutting was observed in this species in a rainforest region in northeast India. The study area, Dampa Tiger Reserve (c. 500 km2, 23°20'-23°4TNand 92°15'-92°30'E), in western Mizoram state, contains an extensive tract of tropical evergreen forest vegetation. Two other species of hornbills, the Wreathed Hornbill, Rhyticeros (= Aceros) undulatus and the Oriental Pied Hornbill Anthracoceros albirostris (incorrectly called A. malabaricus in Ali and Ripley 1987) also occur in the study area and were seen on a regular basis in the rainforest.

On 11 April 1995, while walking a transect in mature rainforest in the Tuichar valley near the Chawrpialtlang range (altitude c. 450 m), four Great Hornbills were spotted. Three of the birds were males and were perched on an emergent Tetrameles nudiflora tree. A female was also perched nearby. At 06h21, one of the males took off from the branch where it was perched, flew out just above another perched male, and while still in flight, clashed its casque loudly with that of the perched male. Flying past the perched male, it then settled on another branch. After a few seconds, it took off from the perch and repeated the behaviour, clashing its casque with the perched male. This performance was repeated several times, until 06h30, when all the birds took off and flew away in the same direction. To all appearances, the other male and the female did not participate in the above interaction. It also should be noted that this observation, where one of the interacting males was perched, is different from that reported for Helmeted Hornbills, where both individuals clashed their casques in mid-air flight (Cranbrook and Kemp 1995).

The observed behaviour may have been a territorial interaction among the hornbills, which had the enhanced yellow plumage colouration developed during the breeding season (Ali and Ripley 1987, R. Kannan pers. comm.). It is intriguing that the interaction was seen between only two of the three males present. It is not known, however, whether the other male joined in the interaction after the hornbills disappeared from view (chased by one male?). Could the male-male aggression have been a form of competition or display for securing the female, as two of the males appeared to be unpaired? Unfortunately, the exact breeding season of Great Hornbills could not be determined during the study. Judging from the observation of plumage and vocalizations, however, it appeared that some initiation of breeding activity may have occurred between late February and May and breeding may have continued after the onset of the monsoon (mid-May to June) after I left the study area. Ali and Ripley (1987) report April- May as the (onset of?) breeding season of this species in the Himalayas. Preliminary observations from Pakhui Wildlife Sanctuary in Arunachal Pradesh also seem to indicate that breeding in the Great Hornbill begins around April-May (A. Datta pers. comm). While more definitive evidence is required, it seems likely that the observed behaviour is thus a pre-breeding interaction between adults.

With regard to the recent placement of Helmeted Hornbills in the same genus as the Great Hornbill, the fact that this rare behaviour has so far been reported from only these two species is significant. A notable difference between the two species is, however, that the Helmeted Hornbill, unlike other Buceros, has a solid casque (vs. hollow casque) that may be better suited to withstand aggressive casquebutting interactions. It would be interesting to discover if such aerial casque-butting behaviour occurs in the other species of Buceros hornbills as well.

The study was supported by a fellowship from the Ministry of Environment and Forests, Govt. of India, and by a grant from Per Undeland through the Oriental Bird Club, U.K. I thank R. Kannan, Suhel Quader, Madhusudan Katti, and an anonymous reviewer for comments and the Mizoram Forest Department and severalofficials and field staff for permissions and assistance.

REFERENCES
Ali, S. and Ripley, S. D. (1987) Handbook of the birds of India and Pakistan. Oxford: Oxford University Press.

Cranbrook, Earl of, and Kemp, A. (1995) Aerial casque-butting by hornbills (Bucerotidae): a correction and an expansion. Ibis 137: 588-589.

Kemp, A. (1995) Birdfamilies ofthe world1. The hornbills:Bucerotijormes Oxford: Oxford University Press.

Poonswad, P. and Kemp, A. C. eds. (1993) Manual to the conservation of Asian hornbills. Hornbill Project. Bangkok: Faculty of Science, Mahidol University.

Raman, T. R. S. (1995) Shifting cultivation and conservation of tropical forest bird communities in Mizoram, northeast India. Unpub!. M.Sc. dissertation, Dehradun: Wildlife Institute of India (Saurashtra University, Rajkot).
Kindly submitted by:

T. R. Shankar Raman, Nature Conservation Foundation
   
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