Fruit-eating Birds Vital To Rainforest

by Wallace Ravven

In a three-year study in a remote Cameroon rain forest, biologists at San Francisco State University and the University of California at Davis have discovered that the toucan-like birds disperse seeds of nearly a fourth of the tropical trees, flying 100 miles or more through the forest in search of ripening fruit. Until this study, the birds were thought to be sedentary, residing throughout the year in patches of rainforest.

As other important seed dispersers such as elephants and primates continue to decline in the region due to habitat destruction and hunting , hornbills become even more important for rain forest regrowth and survival.

"The survival of the rainforest appears to rely to a large degree on the hornbills' ability to disperse seeds of so many species," said Thomas Smith, associate professor of biology at San Francisco State University and co-author of two scientific papers just published on the Cameroon study. "If we have any hope of protecting rainforests we need to protect not just the pattern of biodiversity but also the processes that create it. Our work suggests that by dispersing seeds, these magnificent birds are vital agents of biodiversity.

"The birds' surprisingly large range suggests that their own survival depends on preserving large expanses of rain forest intact," added Smith, who also holds a post as associate professor at the Center for Population Biology, University of California, Davis.

The Cameroon study found that nearly all the seeds dispersed by the hornbills germinate successfully, making the birds one of the prime agents of lowland rainforest regeneration.

The resourceful hornbills may actually be nomads rather than migrants, wandering through the rainforest in search of fruit. Because of the difficulty of following the birds over great distances in the rainforest, Smith and colleagues are starting to work with scientists at the Wildlife Conservation Society and NASA's Jet Propulsion Laboratory to track the birds' movements by satellite. The biologists will attach small radio transmitters on up to 30 hornbills, and NASA will track their whereabouts.

"Hornbills were thought to live in relatively confined rainforest habitats," said Ken Whitney, who led the seed dispersal studies as a graduate student at San Francisco State. "But this research shows that the movement pattern of a few hornbill species may be more like that of elephants and some primates, rather than forest birds."

Whitney, now a doctoral student at UC Davis, is the senior author on papers reporting the hornbill research in the current issues of the Journal of Tropical Ecology and Animal Conservation.

The study found that two hornbill species actively track ripening fruit through the forest. With raucous calls, massive bills, and wingspans of up to four feet, hornbills commonly soar across gaps in the forest in search of ripe fruit. The study found that three hornbill species feed on about a quarter of all trees in the rainforest -- 59 species in all.

"Monkeys and elephants aren't as good at moving across gaps in the forest," says Smith, "so hornbills likely provide a crucial first step in forest regeneration, particularly in regions that have been cleared."

This is the first study to clarify both the hornbills' surprising range and the extraordinarily high numbers of plants for which they disperse the seeds.