New database unveils the role of Asian hornbills as forest seed dispersers

Biologists have long known hornbills are supreme long-distance seed dispersers. The iconic forest birds are capable of transporting tree seeds over vast distances — up to 10 kilometers, or 6 miles, for some species. In so doing, they distribute tree populations across increasingly fragmented tropical forest landscapes. But actually observing hornbill seed dispersal behavior in the wild is notoriously difficult.

“If you’re collecting data in the field, it’s very hard to observe hornbills interacting with plants, not to mention finding out where the bird flies to when it leaves the fruiting tree,” Hanci Liang, a doctoral candidate at the National University of Singapore (NUS), told Mongabay. While a lot is known about the eating habits of hornbills, many mysteries still remain.

To pinpoint where such observational data is needed most, Liang and her colleagues from Singapore and Malaysia have brought together all existing research data on the fruit-eating habits of Asian and New Guinean hornbill species into an open-source digital database.

Published in a new study in Global Ecology and Conservation, the database represents the largest continually updatable repository of hornbill frugivory and seed dispersal research in the region. The team says it hopes the resource will enable hornbill specialists and prospective students to quickly identify where information is lacking so that they can target their efforts and limited resources to fill knowledge gaps.

Understanding the dynamics of seed dispersal enables biologists to investigate the role of different types of fruit-eating animals within ecosystems. And for tropical trees, long-distance seed dispersal courtesy of hornbills is particularly beneficial. It gives seeds distance from the parent tree, which saves it from being crowded out by its siblings or eaten by a herbivore. It also helps tree populations maintain their genetic diversity and move across fragmented landscapes to keep pace with environmental change.

“The kind of interaction data we have on what the birds are feeding on, in what season, which seeds they’re dispersing, how far they’re dispersing them … comes from isolated studies,” Rohit Naniwadekar, a hornbill specialist at the Nature Conservation Foundation in India, who was not involved in the study, told Mongabay. “This new study has brought all this knowledge together very clearly.”

To demonstrate the versatility of the database, the researchers also present an analysis of beak morphology and frugivory data sets to confirm that hornbills are capable of eating and dispersing particularly hefty seeds that far outsize the gape of other avian frugivores.

Jun Ying Lim, an assistant professor at NUS and co-author of the study, said this indicates that hornbills could be the sole dispersers of many large-seeded tree species, especially given hornbills can swallow such massive tree seeds whole without crushing, mushing or otherwise damaging them. “This new database will allow us to better triangulate which of these plant species that uniquely depend on hornbills might be under threat if we lose hornbills,” Lim added.

Island mysteries remain 

To collate the database, the team processed a total of 2,365 records of hornbill-plant interactions from 71 peer-reviewed sources from across the region, comprising information on 26 of the region’s 32 hornbill species and more than 470 plant species.

Just three species of hornbill accounted for 44% of the records: great hornbills (Buceros bicornis), oriental pied hornbills (Anthracoceros albirostris) and wreathed hornbills (Rhyticeros undulatus). The latter were the prime seed dispersers, the study says, documented as eating and dispersing the fruits and seeds of 162 species of plant.

Plants from the fig family (Moraceae) were the most consumed, being eaten by all but one of the 26 hornbill species. However, the dietary records featured many other large-seeded plants, including Myristicaceae, a group of trees that contain nutmegs; Meliaceae, a hardwood family that includes mahogany; and Annonaceae, which yield large fruits like custard apples and soursops.

While the authors uncovered a wide array of plants in hornbill diets, they point to a lack of information about the ultimate fate of the seeds ingested by hornbills. Just 1% of the research records had measured the viability, or ability to germinate, of “deposited” seeds, according to the study.

Information gaps also exist in certain geographic areas. Records were only available for four of the 11 hornbill species that occur in the Philippines, many of which are not only endemic, occurring in no other country on the planet, but also restricted to just a few islands.

The paucity of data on the island-endemic species is worrisome for Naniwadekar, who has studied Narcondam hornbills (Rhyticeros narcondami), a species restricted to its namesake island in the Andaman Sea. He said island ecosystems are particularly shaped by the interactions of plants and frugivores like hornbills compared to the mainland, so it’s crucial to study them.

“Every island ecosystem will have its own story to tell,” Naniwadekar said. “But for many island-endemic hornbill species, we still don’t know how they’re helping shape those island communities. And on top of that, we are losing those hornbills, so we are going to forever destroy something that is so unique and so different. If those interactions are not studied, we don’t even know what we are losing.”  

Besides the Philippines, information was also lacking from several other countries that boast diverse assemblages of hornbills. Myanmar, for instance, is one of the most hornbill-rich countries in the world, home to 10 species of hornbill, yet the research team found no hornbill frugivory studies from the country. Cambodia, Laos and Vietnam were similarly lacking information.

The authors suggest the disparities in research effort across Asia and New Guinea could be down to differences in institutional capacity and funding. Worryingly, it could also reflect intense pressures on wild hornbill populations in these countries. “High past and ongoing hunting pressure and political turmoil, especially in Cambodia, Laos, and Myanmar, may at least partly explain the lack of hornbill research in those countries,” the study says.

Intense pressures

Twenty-one Asian and New Guinean hornbill species are listed on the IUCN Red List as threatened with extinction. Deforestation and habitat degradation devastate hornbill populations, partly because all species require large, old trees with suitable nesting cavities. They’re also captured from the wild for live trade and targeted by poachers for their highly sought-after head casques, a pressure that has led to the extirpation of some species, such as helmeted hornbills (Rhinoplax vigil), from parts of their range.

If hornbills continue their trajectory of decline, it could be disastrous for tropical forests, according to Naniwadekar. “When hornbills are lost, the nature of the forest will change over the next few decades and centuries, because those plants that hornbills are feeding on and dispersing will become filtered out,” he said.

There would also be knock-on effects on forest carbon storage. Studies have shown that large-seeded trees typically have a greater ability to store carbon in their slower-growing, higher-density wood. “With loss of hornbills, you will have a loss of large-seeded plants which will eventually have other cascading consequences on carbon sequestration,” Naniwadekar said.

“For me, the hornbill is a true Carbon Superhero that works silently helping tropical forests to maintain their health integrity [and] to store more carbon,” Yoki Hadiprakarsa, founder of the Indonesian Hornbill Conservation Society (Rangkong Indonesia), who was not involved in the new study, told Mongabay.

As a member of the IUCN’s Hornbill Specialist Group, Yoki said hornbill research in Indonesia tends to be plagued by a lack of funding opportunities. As a result, he frequently sees promising researchers opt to work with what he calls “celebrity species,” like orangutans and tigers, which are typically better funded. He added that the hornbill-plant interaction database is a useful resource that could help fast-track limited labor and funding toward forest conservation and hornbill research initiatives.

A boost for reforestation efforts

The new frugivory database could also prove useful for reforestation projects, many of which increasingly recognize the importance of planting figs and other food plants to attract natural seed dispersers, which in turn help to regenerate the forest further.

John Payne, executive director of the nonprofit Bringing Back Our Rare Animals (BORA) in Malaysian Borneo’s Sabah state, has spent many years rewilding an oil palm landscape with orangutan food plants. “There is a need to demonstrate what plant species are the most important for hornbills and other frugivores,” Payne told Mongabay, commending the “specificity, precision and quantification” of the information in the new database.

Payne, who was not involved in the new study, said clear and up-to-date information about which plants seed dispersers feed on could help reforestation initiatives to avoid wasted money and effort. A prevailing practice that concerns him, for instance, is reforestation initiatives planting common, hardy tree species that would otherwise regenerate naturally.

“Reforestation work should serve to diversify the ecosystem and to favor the natural seed dispersers,” Payne said. “There are many relevant plant genera, but Ficus [the fig genus] is the most obvious, because there are 150 species in Borneo (for example) and they can be propagated not only by seed but also vegetatively in the nursery.”

To support fig-based reforestation efforts, Payne and his colleague at BORA, Zainal Zainuddin, manage the world’s largest collection of figs at the Sabah Ficus Germplasm Centre in Tabin Wildlife Reserve. They also recently compiled a habitat restoration manual focused on fruit-eating wildlife together with WWF-Malaysia.

Just the beginning

The hornbill database is just the starting point for a much larger body of work, according to Lim. He and his team at NUS are compiling similar data sets for other key frugivore vertebrates, including bats, civets, bears, elephants, tapirs and other bird groups like barbets, pigeons, bulbuls and corvids.

“My hope is that once we can put this [data] together, we can answer a lot of very interesting questions,” Lim told Mongabay. “Like how do all these different frugivores coexist with each other? Do they feed on different kinds of resources? And do they have unique or complementary effects on promoting biodiversity in forests?”

While hornbills are clearly crucial to the health of both existing and future forests, their appeal for lead author Liang extends beyond their ecological function. Growing up in mainland China, far from the natural range of hornbills, Liang said her fascination with the giant birds was only sparked last year when she witnessed the pairing bond between a male and female rhinoceros hornbill (Buceros rhinoceros) on the banks of the Kinabatangan River in Sabah.

“We heard a female rhinoceros hornbill calling from one side of the river,” Liang told Mongabay. “And then her boyfriend, or husband, flew from the other side of the river to her. He jumped along the branch to the female and regurgitated all the fruit he had in his crop to feed [her]. It was so amazing, you could really feel something like human emotion between them. They have this intelligence, wisdom.”

Carolyn Cowan is a staff writer for Mongabay. Follow her on 𝕏, @CarolynCowan11.

Banner image: A male oriental pied hornbill eating an Annonaceae fruit in Pulau Ubin, Singapore, in February 2024. Image courtesy of Hanci Liang.

Citations:

Liang, H., McConkey, K. R., & Lim, J. Y. (2024). From beak to fruit: An Asian hornbill database for frugivory and seed dispersal research. Global Ecology and Conservation, 51, e02879. doi:10.1016/j.gecco.2024.e02879

Bello, C., Culot, L., Ruiz Agudelo, C. A., & Galetti, M. (2021). Valuing the economic impacts of seed dispersal loss on voluntary carbon markets. Ecosystem Services, 52, 101362. doi:10.1016/j.ecoser.2021.101362

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