Just before dawn, fifty miles north of Manaus, my assistant and I opened our bird nets in a small section of Brazilian rainforest. We generally snared more than half of each day's bird catch during these cool morning hours. A few miles away, in continuous forest, a glorious cacophony of birdcalls greeted the dawn, but at our fragment site--about the size of two football fields--mornings were ominously silent, for comparatively few birds were then inhabiting such small forest patches.
In the dim Amazonian light, our fine mesh nets, stretched out about seven feet above the ground, were nearly invisible to low-flying birds. Just after dawn we caught several, including an antbird and a common flycatcher, which we identified, measured, and released. Usually we captured few birds in the middle of the day (when the heat seemed to be as oppressive to the birds as it was to us), so I often went for a leisurely walk. This time, as I returned from my stroll at about 11:00 A.M., I heard excited chattering from the direction of the nets. I could make out the calls of several kinds of birds and wondered why there was so much commotion. Then I realized these were all birds that follow army ants to find their food. An ant swarm was probably moving through the fragment.
As I approached the nets, I could hear not only bird vocalizations but also the faint buzz of innumerable insects and the low rustle of tens of thousands of army ants moving along the forest floor. When I reached the nets, I nearly panicked at the scene that confronted me. The ant swarm was moving directly under the nets, creating a dangerous situation for the captured birds, whose weight had made the nets sag. Any bird that touched the forest floor would be attacked. Hundreds of ants ascended my boots as I raced down the line of nets, lifting the bottoms so birds would not touch the ground. The venom from ant bites and stings can easily kill birds and other small animals, but the ants' mandibles are incapable of tearing flesh off a large mammal like me. The ants climbing up my legs turned back after reaching knee level.
Within about two hours, to our relief, the ants were gone from the area near the nets, and I had banded and released seventeen birds. Seven of these were white-plumed antbirds, rufous-throated antbirds, and white-chinned woodcreepers--species that do all their feeding at ant swarms.
In 1979, soon after cattle ranchers started cutting down the forests north of Manaus, researchers began monitoring the bird populations in the fragmentary patches left standing. Within a few years, they found that the white-plumed antbird--the bird that had been the most frequently netted before extensive cutting--was no longer present in the forest fragments. It was still thriving, however, in uncut, continuous forest in the area. But now I had abundant evidence in my nets that the birds were back in their former habitat. Why had they disappeared and then returned?
To understand what was happening, one must consider the behavior of antbirds. For some species of insectivorous birds, discovering an army ant swarm moving through its territory is like winning a free shopping spree at the supermarket. The ants (themselves unpalatable to the birds) feed on insects and other arthropods, which they flush out of the leaf litter as they move through the forests. All the birds have to do is pounce on the exposed, distracted prey.
Several kinds of birds hunt with ant swarms when the opportunity arises, but some birds are "professional" ant followers and can find their food in no other way. Among these specialized species are the white-plumed antbird, the rufous-throated antbird, and the white-chinned woodcreeper, all of which are restricted to the Amazonian rainforest.
Ant following as a specialty has developed only in South America, where several kinds of ants swarm regularly during the day. (The nocturnal swarmers and related ants of Africa and Asia have not developed complex interactions with birds.) But even Eciton burchelli, the most common army ant at our site, forms large swarms for only about fourteen days out of its thirty-five-day reproductive cycle. During the other three weeks, the queen, the pupae, and some of the workers remain secluded, while most workers forage independently. Fortunately for the birds, the various ant colonies are not in phase with one another, so birds can always switch swarms.
Birds that attend ant swarms have developed various techniques to collect their bounty. Woodcreepers perch on tree trunks, while antbirds generally perch on small branches, often at precarious angles. A size-based dominance hierarchy determines which species gain access to the profitable central area of the ant swarm. The reason that we net white-plumed antbirds most frequently is not because of their great numbers but because they are forced to spend more time in flight. Since they are small, they are often chased off by such larger species as white-chinned woodcreepers and rufous-throated antbirds and so must constantly commute among several swarms.
To keep track of the movements of several ant colonies at once, ant-following species range over very large areas in comparison with nonspecialized birds of similar size. Ornithologist Lee Harper, who also works near Manaus, determined that white-plumed antbirds, although only the size of house sparrows, forage daily over as much as one square mile. The birds must go where the ants lead them, and swarms may move a hundred yards a day; over several weeks, the birds may range over more than 125 acres. Time and energy spent commuting probably peak during nesting, when birds must travel back and forth to deliver food to their broods and to nesting females.
Forest fragmentation disrupts these deeply established patterns of behavior. When loggers isolate small patches of forest, ant followers persist briefly in the fragments but then disappear. No one knows whether they starve in the altered habitat or cross the newly cut landscape to the safety of continuous forest. The birds do not, however, follow the ants into open pastures.
Interestingly, the army ants themselves are less affected by logging and will forage in the scrubby fields and fragment edges. Undaunted by open spaces or young, regenerating forest, they often leave the forest fragments at dusk and bivouac overnight at tree stumps in cow pastures. They even swarm fearlessly over the kitchen and sleeping areas of researchers--ours is located in a clearing within the pasture--and do us the favor of exterminating roaches and other undesirable insects. On one occasion, as I watched an ant swarm leave the forest and enter our camp, I took careful notice of the behavior of its avian camp followers. A few ant specialists were attending the swarm, but most did not follow the ants into the tangled vegetation at the edge of the clearing. Black-headed antbirds, however, prefer forest edges, and one of them continued following and feeding at the swarm up to the point where vegetation stopped and our campsite began. None of the birds followed the swarm into the clearing.
Mourning the white-plumed antbird's demise in forest fragments turned out to be premature, however. In the first few years after the forest was cut, some fragments remained isolated in the midst of cattle pastures, while others were soon surrounded by regrowth. Although the new vegetation did not provide sufficient cover for the birds, after about six years of isolation, some of the regrowth had formed a forest with a closed canopy as tall as thirty feet, dominated by rapid-growing Cecropia trees. At that point, ant followers returned. After an absence nine years in some fragments, obligate ant followers reappeared in the same numbers as before isolation, and became common in the overgrown pasture as well.
But the forest in the fragments was unlike the uncut rainforest. The smaller patches were so damaged by fallen trees--a result of exposure and drying out at the edges--that they bore little resemblance to old forest. Without the stabilizing effects of neighboring trees and vine connections, weakened trees fell over. Most of the smaller fragments we studied have become gigantic brush piles. Larger fragments also suffer desiccation at the edges, but the interior trees are protected from winds. Apparently, both army ants and ant-following birds can tolerate these modified forests.
