ABERRANT BEHAVIOUR OF A FEMALE GREAT HORNBILL AND A FEMALE RHINOCEROS HORNBILL

Introduction

Between February and May 2006, a female Great Hornbill (Buceros bicornis) and a female Rhinoceros Hornbill (Buceros rhinoceros) were observed regularly at a patch of secondary forest in Eng Neo Avenue, Singapore. Both are escapees from captivity and are the only individuals of their species in Singapore. Observations indicated that the Great Hornbill acted as the male, while the Rhinoceros Hornbill acted as the female. They were observed prospecting a cavity along the trunk of an old albizia tree (Falcataria moluccana = Paraserianthes falcataria) at this locality. The Great Hornbill regularly fed the Rhinoceros Hornbill as in courtship feeding, and also inserted regurgitated food, probably fruits, into the cavity. The Rhinoceros Hornbill was observed to fly over to check the cavity, occasionally poking its head inside. This article describes their behaviour at the Eng Neo Avenue site from January 2007 to August 2008, and presents the first photographic evidence of the Great Hornbill's occupation of that cavity.

Observations and Discussion

In 2007, at the Eng Neo Avenue site, both hornbills were observed to usually fly in at about 0900 hours, spend some time together, and then fly off. They sometimes returned at around 1030 hours. At times, they even visited in the evenings at approximately 1700 hours. The arrival of the pair was usually announced by their characteristically loud honking.

The pair continued their courtship feeding around the nearby shorter trees, with the Great Hornbill feeding the Rhinoceros Hornbill with regurgitated food. On and off, the Great Hornbill would fly to the cavity of the albizia tree, poke its head in, and pick up some fruits or place some fruits inside. Invariably, the Rhinoceros Hornbill would also fly over. After checking the cavity, the pair would usually indulge in preening for some time before flying off. Should one remain, the other would call to it, and there would be loud duetting until both were together, to fly off to another destination.

Figure Descriptions:

• Figs. 1-2 show the female Rhinoceros Hornbill (top) and the female Great Hornbill (bottom) inspecting the albizia tree cavity.
• Figs. 3-11 document the stages in the emergence of the Great Hornbill from the albizia tree cavity. This sequence shows the tip of the bill appearing first, followed by the large and prominent yellow casque. As the head slipped out, the body followed, and the right wing abruptly unfolded upwards. The bird then lunged downwards as the right wing made a downward flap to facilitate the left wing's emergence.
• Figs. 12-13 illustrate an instance where the Great Hornbill was inside the cavity, and the Rhinoceros Hornbill flew over from a nearby tree and interlocked its bill with that of the Great Hornbill. A loud "quack" was heard when bill contact was made.

Under normal circumstances, a male hornbill induces a female bird to enter a cavity during the breeding season. Once the female bird approves of the cavity, it will begin to seal itself inside with mud. It will then lay its eggs, and the male will regularly feed it until the chicks fledge.

The birds were regularly observed at the nearby Bukit Timah Nature Reserve. In January 2007, when a large fig tree was fruiting, both hornbills visited daily and indulged in courtship feeding.

At night, the birds were observed to roost elsewhere. The Great Hornbill was spotted for months spending nights alone in a rain tree (Samanea saman) near a condominium along Adam Road, arriving around 1900 hours and staying all night. At around 0645 hours, it would start stretching its wings and preening before flying off towards Eng Neo Avenue. Toward the second half of 2006, the Great Hornbill moved elsewhere to roost and has not returned to its previous roosting site. Thus, the two hornbills would spend the night separately, meeting only during the day. The roosting location of the Rhinoceros Hornbill remains unknown.

From January to August 2008, both hornbills were observed visiting the site at Eng Neo Avenue irregularly. The albizia tree has now died, leaving only the trunk standing. Soon, the hornbills will have to seek out another tree with a suitable cavity for their courtship rituals, as this rotting trunk will soon collapse. The birds continue to visit the nearby Bukit Timah Nature Reserve whenever the fig trees are fruiting.

These birds have been a pair for some years, and their aberrant antics are worth monitoring further.

Acknowledgements

The authors would like to thank Brian Ng for alerting them to the roosting site of the Great Hornbill and for providing details of its behaviour. All images are by Chan Yoke Meng.

Literature Cited

• Kemp, A. C., 1993. Conservation of Asian hornbills and their habitats: An introduction. In: Poonswad, P. & A. C. Kemp (eds.), Manual to the Conservation of Asian Hornbills. Hornbill Project, Thailand. Pp. 4–23.
• Tsuji, A., 1996. Hornbills: Masters of tropical forests. Sarakadee Press & Hornbill Research Foundation, Bangkok. Pp. 108.
• Wee, Y. C. & R. Subaraj, 2006. Aberrant behaviour of a pair of female great and rhinoceros hornbills in Singapore. Birding Asia, 6: 18–22.