“That is the call of the great hornbill,” our Nepal bird guide whispered reverentially as raucous barking rang through the forest, followed by the swooshing sound of the three-foot-long birds flapping toward a new perch far overhead. Spotting the birds was easy: Their bold black-and-white bodies, topped by bright yellow casques above huge yellow beaks would have made them conspicuous even if their noisy behavior hadn’t revealed the pair’s location high in a bombax tree.
Great hornbills (Buceros bicornis), the largest of Asia’s hornbills,are spectacular looking. A glance through the color plates of The Ecology and Conservation of Asian Hornbills: Farmers of the Forest reveals that they share that quality with many of their relatives, 30 other species in Asia alone. (Another 23 species live in Africa, including the red-bellied hornbill, Tockus erythrorhynchus, and Von der Decken’s hornbill, T. deckeni, seen in the Zoo’s Bird House.) But, as they say, looks aren’t everything. As Margaret F. Kinnaird and Timothy G. O’Brien learned in the course of nearly 15 years of exhaustive research on a variety of species in the “hornbill realm” of tropical Asia, everything about these “forest farmers” is pretty spectacular.
Take their very unusual breeding behavior. Asian hornbills live in pairs that stay together year round. The breeding season (which varies with the species) begins with about a month of courtship that culminates in mating. Immediately after, the female hops into a tree hole and begins walling herself in. Using fruit mash, her feces, and sometimes mud to form cement, she uses her bill like a trowel to apply layers of sealant until only a thin slit connects her to the outside world. And here she stays while she incubates and during some or all of her chick’s pre-fledging nestling period—an average of 92 days for all Asian hornbills and as long as 175 days in one species. At the end of this long period of isolation, she breaks out using her bill like a chisel! Throughout this time, her faithful mate nourishes her and their chick with regurgitated food through the nest hole’s small opening.
Kinnaird and O’Brien review the various explanations that have been proposed for this bizarre behavior—that males forcibly imprison their mates, that it protects mother and chick from predation, and that it shelters them from wind, rain, and temperature extremes—and find them wanting. (The first, of course, is patently wrong.) They suggest instead that the female’s self-imposed captivity captivates her mate, ensuring his fidelity because he is too busy feeding her, their young, and himself to fool around, far less support another family.
Intriguing behavior aside, and there’s much more of that described in Asian Hornbills, these birds are critical to the forests in which they live. Asian hornbills are largely fruit-eaters. Depending on the species, fruit may compose from two-thirds to nearly all of their diet: Nesting hornbills down the fruit equivalent of 20 to 33 percent of their body weight daily, and that’s the “equivalent of a 150-pound person consuming 30 to 50 pounds of fruit each day!” As a result, it has long been assumed that hornbills are important dispersers of fruit seeds in the forests where they live. Kinnaird and O’Brien actually carried out the painstaking research required to test this assumption, with clear results: Hornbills well deserve the sobriquet “farmers of the forest.”
Further, they looked at what happens when hornbills disappear from small isolated forest fragments, which more and more of Asia’s tropical forests are becoming. On one Indonesian island, for instance, they found that large forest fragments with resident hornbills boasted 14 fruit tree species in six families that hornbills feed on versus small fragments, which had only six species in two families. Not surprisingly, forest loss and fragmentation threaten the survival of hornbills as well, as does hunting. At least two-thirds of hornbill species are listed as near threatened to critically endangered on the World Conservation Union’s IUCN Red List of Threatened Species, and only a third hold the status of least concern.
One of the most important messages of the book, though, is that the current conservation crisis is not just about the final extinction of charismatic animals such as hornbills. Instead, the authors write, “...it is the chronic whittling away of wildlife populations and the slow but subtle unraveling of the complex interactions among species and between species and their environments that is ominously changing our world.”
The Ecology and Conservation of Asian Hornbills is not easy reading. Kinnaird and O’Brien make few concessions to a non-specialist audience. But if you’re curious about hornbills, about the state of Asian forest diversity, or about how meticulous scientists learn about the natural world, your time reading this book will be well spent.