Hornbills create orchards in Arunachal’s tropical forests

Hornbills are large, endangered, fruit-eating birds found across Asian forests with only certain fleshy fruit trees and yet, they are responsible for dispersing seeds in large patches, thereby creating fruit orchards, a new study has revealed.

India has nine species of hornbills, five of which are found in the Namdapha Tiger Reserve in eastern Arunachal Pradesh. In this vast biodiversity hotspot, forest patches that have rare and large-seeded trees, like Canarium (called Dhoona in Assamese) and Phoebe (called Bonsum in Assamese), act as orchards by attracting certain hornbill species, like Wreathed hornbills, in large numbers.

Scientists from the Nature Conservation Foundation (NCF), Mysuru, and Centre for Ecological Sciences, Indian Institute of Science (IISc), Bengaluru, studied how fruit bearing plants and hornbills influence the distribution of each other in the Namdapha Tiger Reserve, one of India’s most biodiverse Protected Areas.

According to a study published online by the ‘Journal of Avian Biology’ on October 8, hornbills help in dispersing large number of seeds in areas with rare trees. They disperse up to 12,000 large seeds per square kilometre in a day, thereby creating fruit orchards.

“Hornbills are among the very few birds that can feed on fruits with large seeds, regurgitate and disperse the undamaged seeds away from the mother plant. This is an important service hornbills provide to trees. Under the parent plant, lots of seeds/fruits fall, which attract pathogens, insects and rats that destroy most seeds,” lead author of the study, Rohit Naniwadekar said.

“Our study shows that forest patches that have these rare trees attract hornbills in large numbers. In turn, hornbills end up dispersing seeds of a diverse array of plant species in higher numbers in these patches with some of these hornbill food trees. In the long term, this likely creates orchards that continue attracting hornbills,” he further said.

The team conducted the study on the Hornbill Plateau in Namdapha Tiger Reserve across 24 hectare patches. They counted 815 hornbills, 157 hornbill food plants, 946 seeds that were dispersed in 1,600 one-metre and 5,173 regenerating saplings of multiple large-seeded hornbill food plant species.

The researchers commonly observed four species of hornbills that includes the Great, Rufous-necked and Brown, apart from the Wreathed hornbills. The most common Wreathed Hornbill was seen in patches with the rare, large-seeded canopy trees.

“The number of dispersed seeds was the highest in patches with the highest abundance of hornbills. The diversity of regenerating saplings was also highest in those patches,” the study said.

Stating that this study establishes the linkages between hornbill numbers, the abundance of some of their important food plants in certain patches, the seeds that arrive due to hornbill dispersal and subsequent establishment of seedlings/saplings in such forest patches, scientist from NCF, Aparajita Datta, said: “Earlier studies on seed dispersal by hornbills have focussed on what happens at particular locations such as nest and roost sites. This study examines what hornbills do in a scenario when they are ‘scatter-dispersing’ seeds while foraging in the daytime.”

Hornbills are the farmers of the forest as they farm their own food-rich patches through their seed dispersal.

“What may appear as a random assortment of trees may be an orchard created by generations of hornbills. In the lifespan of the long-lived hornbills, it is not improbable for a bird to feed on fruits of a tree, whose seed it would have dispersed,” Datta said.