Acre for acre, a rich New Hampshire mixed-hardwood forest supports as many nesting pairs of birds as a mature rain forest in the Peruvian Amazon. That statement may seem surprising. Tropical rain forests are supposed to be the richest environments on earth, so shouldn’t they have more birds than temperate forests?
Well, yes and no. According to a study published in the journal Ecological Monographs, the richness of the rain forest lies not in the total number of birds but in the number of species and in the size of the birds.
The study was conducted on a roughly triangular, 240-acre piece of mature forest that borders a lake called Cocha Cashu on one side, the Manu River on the second side, and more forest on the third. The site is the location of the Cocha Cashu Biological Station in Peru’s Manu National Park.
Five authors share credit for the article. The senior author is John Terborgh, a professor at Princeton. The others are Scott Robinson, now with the Illinois Natural History Survey, Ted Parker, Charles Munn, and Nina Pierpont. Their study is nearly unique in the literature on tropical forests. Earlier studies of rain-forest birds were conducted on very small plots–10 acres or less–and often in areas where heavy hunting pressure by local people had eliminated some species and substantially reduced the populations of others. We are, in fact, in danger of doing with the tropical forests exactly what we did with the forests and prairies of eastern North America: destroying them before we even have a chance to find out what they are made of.
The difficulty in studying tropical-forest birds in their native haunts has been that nobody could identify them without shooting them first. Twenty years ago there were no field guides to the birds of Amazonia, and any scientist who wanted to study them was on his own with very little information to help him.
Things have changed. We now have guidebooks for major portions of the Neotropical forests. Equally important, we have a considerable fund of experience gained by both amateur and professional ornithologists, experience that has taught us much about the appearance of rain-forest birds and about their behavior and their songs and calls.
The five authors of the Cocha Cashu study were all birders who became scientists and then applied their birding skills to their professional work. For a time, Scott Robinson and Ted Parker were each holders of the North American Big Year record. A Big Year is a birding game. The idea is to see as many birds as possible in a calendar year in a given region. Robinson and Parker did that in all of North America north of Mexico.
So these five hotshot birders went to work at Cocha Cashu. Using their experience as well as recordings made by others, they all learned the songs and calls of more than 500 tropical-forest species. “This skill,” they write, “is the cornerstone of our work and a sine qua non for studies at the community level.” Thanks to this skill, they became the first scientists to apply the spot-mapping technique–standard in temperate-zone studies–to a large plot of tropical forest.
Walking a system of trails laid out at 200-meter intervals, they could record–spot map–the locations of all the birds they saw and heard. They supplemented these walks by searching for nests and by setting up mist nets to capture and band birds. They censused nearly every day during the breeding season, which runs from mid-August to mid-November. They started their morning walks about 15 minutes before the first gray light of dawn showed on the eastern horizon, and they did another survey at sundown, when some species sing again.
They discovered about 1,000 breeding pairs on their 240-acre plot, about the same number you would expect to find in a rich forest in eastern North America. However, there the similarities ended. In a North American forest those 1,000 pairs would belong to a total of about 50 species. In Peru the 1,000 pairs represented 245 species, a number approximately equal to all the species known to breed in the entire state of Illinois. Biomass–the aggregate weight of all those birds–equaled 190 kilos per square kilometer, also about five times what you would find in a North American forest.
The biomass figures are skewed somewhat by the ecological disaster of European settlement of North America. Most of our eastern forests no longer have wild turkeys, and that lowers the avian biomass considerably. And we no longer have passenger pigeons, once the most abundant bird in our eastern forests. For that matter, we no longer have American chestnuts, once the most abundant producers of mast in the eastern forests.
The species list for Cocha Cashu makes it easy to see why every North American birder would love to visit the Amazon. For example, we have eight species of falcons in North America. Seven species nest on 240 acres at Cocha Cashu. And there are 14 other species of hawks and eagles, ranging from the enormous harpy eagle down to the tiny hawk, a forest accipiter that looks like a goshawk that has been shrunk to the size of a robin.
Cocha Cashu also has 18 species of parrots, including six different macaws. There are 14 species of hummingbirds, 12 species of woodpeckers (we have 20 in North America), and 24 species of tanagers (we have four in North America). And then there are all the exotic glories of the rain forest that simply do not occur at all in our icy latitudes: motmots, jacamars, puffbirds, barbets, toucans, tapaculos, manakins, cotingas.
Their ways of life are as exotic as their names. Here in the U.S. our forest birds are solid bourgeois types, potential presidential candidates who live in monogamous pairs in precisely defined territories. There are birds like that in the rain forest too, but there are also polygynous types and species that live year-round in flocks. Some flocks are all of one species; some are mixed. Some stable mixed flocks contain exactly one pair of each of several species, and these birds nest and feed together.
Parrots move around in single-species flocks. If you are standing in the woods, it is very difficult to count the passing parrots as they move through the trees. So to census the psittacines, Scott Robinson sat in a canoe on the lake and counted them as they passed over in the open sky. The average size of these flocks was taken to be the average size of flocks in the woods, so parrot numbers were computed by multiplying flock sightings by the number of individuals in the average flock.
Here in North America singers are almost always males, but in the rain forest females may sing too. In some species the genders are identical in appearance and song, making it very difficult to tell how many nesting pairs are present.
Tropical forests also have numerous species of ant followers. Columns of army ants moving along the forest floor scare up large numbers of insects, and these birds hang around the ants and eat the insects. Since we have no army ants in our woods, we also have no ant followers.
The difference between the biomass of the tropical bird community and the biomass of temperate-forest communities was largely accounted for by the presence in the tropical forest of a small number of large seedeaters. Birds like the tinamous, terrestrial seedeaters that fly only to escape danger or to reach roosting perches in the trees, are the biggest of these. At Cocha Cashu there are nine species of tinamous, the biggest of which is the gray tinamou, which weighs about 4.5 pounds on average. These birds are favorites of hunters and often disappear from forests that are too close to large human populations.
Insectivores, often the dominant group in temperate forests, account for only about 18 percent of the biomass in the tropical forest. Tiny insect eaters like the warblers and vireos of our woodlands are unusual in the tropics. This may reflect the differences in insect communities. Here the favorite foods of warblers are the soft-bodied caterpillars of butterflies and moths. In the tropics the most common insects are hard-shelled beetles, so insect eaters have to be big and strong in order to crush the shells.
Tropical animals are often described as rare, and the Cocha Cashu study tends to support that idea. Territories tend to be larger than they are in the temperate zone, so individuals are more widely scattered even in prime habitat.
Another intriguing finding is that population density reached its peak in the forest interior, not at the edges. Fragmenting the tropical forest would almost certainly lead to a loss of species.
Work such as the survey at Cocha Cashu represents a beginning. We are starting to penetrate the mysteries of the most complex ecosystems on earth. We will never learn all there is to know, but we can hope that the forests will be there in the future so that we can at least continue our studies.