White-throated Brown Hornbills: Cackling in Assam’s Lowland Rainforests

It was an afternoon in mid-January, and the rainforest was silent but for the continuous sound of the crickets, cicadas, and the occasional clicking calls of a black giant squirrel. Suddenly, we heard inane chuckles that transcended into a variety of loud squeals, cackles, and piercing yawps coming from high up in the canopy of emergent winged-seeded (Dipterocarpus) trees. The shrieks became louder as we went closer; they didn’t sound like anything we had heard in a month of walking in the dense rainforests of Upper Brahmaputra valley in Assam. Our field assistants confirmed that these calls were of the white-throated brown hornbills (Anorrhinus austeni), birds we had been on a lookout for since our first day in these forests. There they were, at least four of them, perched on the higher branches of an acorn (Castanopsis) tree. I finally had a clear glimpse. We watched them as they preened and regurgitated seeds of fruits they must have eaten earlier in the day, until they swiftly flew away, almost disappearing into the forest canopy. The forest was still again, but their calls echoed in our ears for a long time after.

The grandeur of the rainforests is only truly revealed when witnessed first-hand. For me, this opportunity came when I set out to spend five months studying the interactions between fruiting plants and fruit-eating birds in Dehing Patkai National Park and its surrounding rainforest fragments. The trees of this region stretch up to 40-50 metres in height, with multilayer canopies dominated by hollong (Dipterocarpus macrocarpus), mekai (Shorea assamica), and nahor (Mesua ferrea) trees towering over an understorey of climbers, epiphytes, canes, ferns, palms, bamboos, and shrubs.

The distribution of brown hornbills is naturally restricted to the plains and foothill forests up to 1,000 metres above sea level, south of the River Brahmaputra in Assam, Arunachal Pradesh, Nagaland, and Manipur. Moreover, their primary habitat has undergone severe fragmentation besides conversion for tea plantations and agriculture, coupled with oil, coal, and timber extraction over the past century. Consequently, brown hornbills now persist only in pockets of forests and are considered the most threatened of all hornbills in terms of the total population in India. Globally, they are sparsely distributed across lowland forests in Southeast Asia.

I got several chances to closely observe their secretive lives in the subsequent months of my stay. One morning, the forest was filled with the songs of birds and hoolock gibbons. As the mist cleared and sunlight peeped through the dense canopy, I had the most memorable sighting of a flock of brown hornbills. I kept a mental map of the trees that bore ripe fruit along the forest trails I frequented. Navigating along these trails, I always paused under a cluster of Beilschmiedia assamica trees that were fruiting plentifully, and I had often hoped for a sighting that finally came true that morning.

The hornbills were quieter this time, but I had become familiar with their repertoire of calls. Even the gentlest four-note giggles and soft squeals betrayed their presence. I took a suitable vantage point and waited. Soon, a flock of about 14 restless brown hornbills comprising males, females, and juveniles appeared and hastily raided the fruiting trees. They were constantly in motion, hopping and fluttering from between branches, taking small flights, dashing and darting to-and-fro between the fruiting trees — plucking, tossing, and gulping every ripe fruit they encountered. This feasting spree lasted about half an hour, after which the flock took a short rest break — pairs huddled up together while the others sat fluffed up separately on different branches of the same tree. The leader whistled, and they all launched themselves in quick succession into surprisingly noiseless flight.

  

In contrast to the larger, better-protected forests of Dehing Patkai, the smaller reserves continue to face degradation and high levels of anthropogenic pressures. We encountered firewood collection, large-scale logging, and several instances of children attempting to hunt birds using catapults. The forest canopy was open, and biodiversity was noticeably reduced. Sadly, brown hornbills were absent from many reserves, and those that did manage to persist in fragments looked distressed and constrained by a lack of resources like food and nesting trees.

But all hope is not lost. In early April, despite the high humidity, hornbills brim with activity. Summers mark the peak fruiting season for many tropical trees, coinciding with the initiation of hornbill breeding. On an evening walk in a forest fragment, a large, bright-yellow fruit with a pink covering lying on the forest floor caught our attention. My field assistant, Khageshwar, excitedly told me it was “amul” (Horsfieldia kingii). These trees are considered “mast fruiting species”, i.e. they fruit in masses once every couple of years. I was lucky to have witnessed the tree bearing ripe fruits. We spotted two more of these trees bearing ripe fruit in the vicinity. The next morning’s task was set almost intuitively — to check if hornbills come to feed on the fruiting Horsfieldia trees. What followed was a series of fortuitous coincidences.

We got what we hoped for. Sitting on one of its branches, a male brown hornbill gently plucked the yellow fruit, half the size of its beak, out of its cover and swallowed it whole. What we witnessed next was remarkable.

The male hornbill had flown to a nearby tree and sat next to a female and three others. Our eyes fixated on the pair through a small gap in the canopy. Soon enough, the male flew to a trunk of a large hollong tree and perched vertically against a hollow not very high above the forest floor. The female followed and settled next to its partner, both carefully inspecting the hollow’s features as if measuring the dimensions of their probable nest cavity. Almost immediately, the female squeezed inside the compact walls of the cavity while the male perched right behind her, like a concerned guardian. The female then turned around and peeped back from within the hollow. This event lasted about 15 minutes and gave us goosebumps throughout. In subsequent days we saw an array of regurgitated large seeds and remains of animal matter under the nest tree, indicating that the inmates were well-fed.

It was heartening to see brown hornbills breeding even in degraded forest fragments and persisting despite high human-induced disturbances. This was true not just for the hornbills but for several other species of that region, from elephants to gibbons, that face daily threats to their survival.

While certain areas in the landscape are well-protected, the future of many rainforest patches and the long-term survival of brown hornbills remains perilous. Improved protection and active ecological restoration (by planting native fruiting plants in degraded fragments with the support of local communities) must be pushed to preserve these fragments and the biodiversity they represent.