How far do hornbills disperse seeds?

Hornbills disperse more than 90 percent of the seeds of the fruits they consume, and the larger-bodied great hornbills and wreathed hornbills can disperse seeds more than 10 km away from the parent tree, a recent study has found.

This is the first study that estimates long-distance seed dispersal by Asian hornbills.

Hornbills, being largely frugivores, primarily eating fruit, are pivotal for seed dispersal. They are often known as ‘farmers of the forest’ due to their propensity to consume fruits and disperse seeds away from their feeding grounds.

However, how far hornbills disperse seeds in Asian forests was not known.

A team of researchers from the Nature Conservation Foundation (NCF), Centre for Ecological Sciences of the Indian Institute of Science, Bengaluru and Irstea in France set out to find the answer.

“This study is important because we have all long wondered how far hornbills disperse seeds in Asian forests,” says Pia Sethi, Senior Fellow of the Centre for Biodiversity and Ecosystem Services at The Energy and Resources Institute.

“We now know that the hornbills, as long suspected (but without any real evidence so far), travel and disperse seeds over long distances and hence are critical to forest dynamics, particularly for large-seeded species,” she added. Sethi wasn’t involved in this study.

Seeds of trees are dispersed either by living organisms like birds or animals or by natural elements like wind or water. Seed dispersal is vital as it ensures the propagation of trees. It also enables the trees to occupy new areas, thereby increasing green cover.

The term seed dispersal entails multiple processes: the number of seeds dispersed, how the dispersal agent (hornbills for instance) handle them, the passage of seeds through the bird’s body, the distance they are scattered away from the parent tree and the nature of the site where the seed falls.

Among these factors, estimating the distance a seed travels from its parent tree is difficult.

Hornbills and seed dispersal

There are nine species of hornbills in India. Of these, five occur in the northeastern states.

The research team zeroed in on two species of hornbills – the great hornbill and wreathed hornbill. The team conducted this study in Pakke tiger reserve in Arunachal Pradesh.

To gauge how far hornbills scatter seeds, the research team led by Rohit Naniwadekar, a scientist at NCF, observed the visits hornbills made to trees bearing fruits, both during breeding and non-breeding seasons. The team then estimated the time it took for the seeds to pass through the guts of the birds. Finally, using the GPS sensors tagged to selected birds — five great hornbills and one wreathed — they could estimate the distances the hornbills moved in the forest.

Using the observations, the researchers could estimate the number of seeds breeding hornbills dropped under the trees they were nesting in and the seeds they strewed away from the nest. They could compare the seeds dispersed by wreathed hornbills and great hornbills, and within great hornbills, breeding and non-breeding males.

The researchers focused on male hornbills. When hornbills breed, the males ferry food for the female and the growing chick. The females incarcerate themselves for several months in a tree-hole to raise the chick. They regurgitate seeds that they feed on during this locked-in state. But with no space for them to disperse the seeds, the females drop them under the nesting tree. These seeds clumped under the tree are easy pickings for many animals of the undergrowth — as are the saplings that burst out of these seeds. Eventually, this leads to poor propagation, and hence the effect of seed dispersal is limited.

The case of males is different. The movement of breeding males is constrained around their respective nests as they regularly visit to feed the nesting female and the chick. So they have smaller home ranges in the breeding season as compared to non-breeding males.

“The mean daily distances covered is lower in the non-breeding season as compared to the breeding season,” according to Naniwadekar.

The breeding males travel more in a day in search of food for their feeding duties than their non-breeding counterparts. Thus, they scatter seeds over a wider area than the non-breeding males.

“Through this, we are able to demonstrate that even within species, the context might determine how the seed dispersal happens,” said Naniwadekar.

The researchers observed 1025 hours of hornbill visits to 161 trees of 21 food sources in the breeding season between February and June over three years. From November to January,  the non-breeding season, the team observed 391 hours of hornbill visits to 62 trees covering 15 food varieties.

These observations recorded overlapped with the movement data for six hornbills. To find out the time it took for seeds to pass through the hornbills’ guts, the researchers fed 823 fruits of five large-seeded trees.

In the breeding season, hornbills were most active early in the morning, but there was also some activity throughout the day. In the non-breeding season, hornbills fed between 5-6 a.m., 8-10 a.m., and after 2 in the afternoon. This indicates, in the breeding season, hornbills make more trips to the fruiting trees to carry food back to the holed-in female. On average, it took 133 minutes for the seeds to pass through the guts of the birds.

The distances of seeds dispersed by breeding great hornbills, non-breeding great hornbills and breeding wreathed hornbills differed. The furthest seeds dispersed by great hornbills were more than 2.5 km, while for non-breeding great hornbills, it was more than 12 km. Breeding wreathed hornbills dispersed seeds more than 10 km away from the tree they feed on.

In terms of the number of seeds scattered away from the tree hornbills feed on, 95 percent of the seeds were dispersed away by breeding great hornbills, whereas non-breeding great hornbills dispersed 89 percent. The wreathed hornbills were even more successful,  dispersing 98 percent of the seeds away from their feeding grounds.

“This indicates that hornbills are very good dispersers as they take the seeds away from the parent (feeding) plants and thereby increasing the probability of the seed to survive,” said Naniwedekar.

“This is for the first time that we have estimates of how far Asian hornbills are dispersing seeds. This study showcases the critical role hornbills are playing in seed dispersal, especially long-distance seed dispersal,” he added. “Most seed dispersers are not long-distance dispersers highlighting hornbill’s role in these ecosystems.”

The researchers found four of the six tagged birds left the protected areas and visited the adjoining reserved forests to feed.

“It reiterates that hornbills need and use large areas,” said  Aparajita Datta, scientist with NCF and co-author in the study. At such times it is possible that hornbills deposit seeds in non-protected areas. “This can bring back forests in degraded areas, provided the human disturbances can remain minimal or under control.”

Datta reckons hornbill populations would need permeable landscapes outside forests to grow, breed and persist. “Hard boundaries outside protected areas would be detrimental to long-term healthy populations as they would confine populations inside limited areas.”

CITATION:

Naniwadekar, R., Rathore, A., Shukla, U., Chaplod, S., & Datta, A. (2019). How far do Asian forest hornbills disperse seeds?. Acta Oecologica, 101, 103482.

Banner image: A hornbill carrying food to feed chicks. Photo by Angadachappa / Wikimedia Commons.