Hiding under trees for hours, counting fruits and visitor birds and by studying their feeding behaviours, researchers from Nature Conservation Foundation, Mysore, have mapped the different frugivore (fruit eater) birds and their interactions that are important for the forest ecosystem.
The study carried out in Pakke Tiger Reserve in Arunachal Pradesh noted that hornbills, one among the large-sized frugivores, are the top seed dispersers. Sadly, they are also the most threatened. This is because they are hunted for meat, and the tribal communities use their feathers for head dresses.
The study looked at 43 tree species, 48 frugivore bird species that were seen visiting them.
A single bird species could visit different tree species, and a single tree species would be visited by different bird species. So a complex network of over 400 interactions was created and studied.
The trees were classified into small-, medium- and large-seeded. The large-seeded trees mainly depended on hornbills and imperial pigeons for their dispersal. The medium-size seeded trees were visited by bulbuls, barbets along with hornbills and imperial pigeons. Though the frequency of visits was similar for all four bird species, the number of fruits removed from trees was high for hornbills.
“Among the different bird species, hornbills were found to be the most effective seed dispersers. They were found to swallow and disperse most of the fruits they handled. They also removed maximum number of fruits — and therefore seeds — in every visit to a fruiting tree,” explains Dr. Rohit Naniwadekar, first and corresponding author of the study published in the Journal of Animal Ecology.
“They swallow the fruit as a whole causing no damage to the seed. They are known to disperse seeds far away from the parent plant and our previous studies have shown that they can disperse up to 13 km.” adds Dr. Naniwadekar.
“We visited a tree during the fruiting season, hid under it and noted the number of birds and different species that visited. We also noted the time they spent on the tree, number of fruits they ate and dropped,” explains Saniya Chaplod, research affiliate at the foundation and one of the authors of the paper.
“Though bulbuls visited small and medium-sized seed trees in large numbers, they did not handle the seed well. They often peck on the pulp and/or drop the seed under the parent plant. But even for large, medium and small-seeded plants, hornbills swallowed as a whole and thus were the best fruit-handlers among the frugivores,” Chaplod says.
Previous studies by the team also noted that when the number of hornbills decreased in an area, the regeneration of large-seeded plants that were primarily dispersed from them was also affected.
“Seeds that fall under the parent tree face heavy competition, predation by rodents and insects and fungal infections. So their chances of survival are very low. Plants depend on frugivore birds to disperse the seeds at favourable sites, which have low competition and predation pressures, to expand their geographic range. And so the decline of frugivores could severely affect the ecosystem.” adds Dr. Naniwadekar.