BARRO COLORADO ISLAND, Panama -- Here in the exuberantly dour understory of the Panamanian rain forest, the best way to find the elusive and evolutionarily revealing spotted antbird is to stare at your boots.
For one thing, if you don't tuck in your pant legs to protect against chiggers and ticks, you will end up a color plate in "Rook's Textbook of Dermatology." For another, sooner or later -- O.K., much later, many, many hiking hours later -- you will finally step into a swarm of army ants boiling out across the forest floor.
At that point you should step right back out of the swarm and start looking for the characteristic flitting and hopping of the thrush-size antbird, listening for its vibrato "peee-ti peee-it" call. Because wherever there are army ants out on a hunting raid, peckish antbirds are almost sure to follow.
The birds are not foolish enough to try to eat them: Army ants are fiercely mandibled and militantly cohesive. Instead, they hope to skim off a percentage of the ants' labor, by snatching up any grasshoppers, beetles, spiders or small lizards that may jump to the side in a frantic attempt to elude the oncoming avalanche of predatory ants.
It's a gleeful reversal of the conventional notion of parasites as little, ticky things that plague large, poorly dressed hosts. Here the big vertebrates are the parasites, freeloading off insects a fraction of their size.
And the parasitic strategy is so irresistible that according to recent research in the journal Ecology, the spotted antbirds on Barro Colorado Island just may be taking it professional. Whereas the species has traditionally opted for a mixed approach -- filching from ant swarms but also finding food on its own -- the island-bound antbirds appear to increasingly depend on army ants to scare up their every meal.
Janeene M. Touchton a researcher associated with the Smithsonian Tropical Research Institute and Princeton, and the principal author of the report, is now trying to identify the personality traits that may facilitate a spotted antbird's leap from amateur to polished parasite. Is it boldness, aggressiveness, a love of novelty? Or maybe a lack of aggressiveness, a nonchalance about territory and a refusal to pick a fight? She is collaborating on the project with Martin Wikelski of the Max Planck Institute for Ornithology.
Dr. Touchton, who is 37, looks as if she could be Keira Knightley's sister and has the field-hardiness of a Dr. Livingstone. In her view, studying spotted antbirds offers an extraordinary opportunity to catch evolution on the wing, to identify the precise steps behind the great mystery of how new species arise from old ones.
"If spotted antbirds on the island are losing some of their plasticity, if they're trading in the life of a generalist for that of a specialist, I want to know what that process is," Dr. Touchton said. "I want to see it happen."
Michaela Hau of the Max Planck Institute and the University of Konstanz, who has studied breeding patterns among antbirds, said Dr. Touchton was "getting fantastic data out of the system -- and she's gathered the data through heroic fieldwork, through crazy, really hard work."
The antbird story also demonstrates the vividly baklavaian nature of parasitism in a tropical rain forest. Researchers have identified three species of butterfly that specialize in following antbirds. The reason? The butterflies feed on bird droppings, and though guano is a notoriously unpredictable resource in a rain forest, these butterflies know where their suppliers are likely to be found.
"I always end my talks by showing a slide of this complex, quadruple-tiered relationship," said Joseph Tobias of the University of Oxford, who studies antbird song and has worked with Dr. Touchton. "You have the ants themselves, followed by a gaggle of antbirds, and behind the antbirds are the butterflies, and behind them are a couple of bird watchers."
Antbirds belong to a large, old and almost purely tropical avian family of some 200 species, only a fraction of which have anything to do with ants. The new research looked at three swarm-stalking species that live in the same region of Panama: the spotted antbird, the slightly larger bicolored antbird and the even larger ocellated antbird. All three adhere to the basic script of tropical birds, living much longer than the two or three years of the average chickadee or sparrow up north but brooding significantly smaller clutches per year, and with males and females singing jointly to prove to the world that they're a formidable team.
"We call them mutually vocally ornamented," Dr. Tobias said. "It's a very egalitarian setup."
All three types of antbirds are adapted to the clotted, shadowy conditions of the understory, which means they flit more than they fly and they avoid open, sunny places. Fields, rivers and hills are essentially impassable barriers.
The birds also have features designed for the ant-following trade -- big, strong legs and long claws for gripping onto vertical stems and branches as they lean sideways to scan the forest floor for ants, and pie-plate eyes to spot minor movements in very low lighting. They look for the earliest possible signs of approaching Eciton burchelli, the most formidable species of army ant in the neotropics.
"Eciton are like lions," said Michael Kaspari, an ant expert at the University of Oklahoma. "They're the top predators of the tropical forest."
Their armies are shape-shifting nation-states built of hundreds of thousands of ants -- bridges of ants, spiraling staircases of ants. Army ants don't dig nests, as most ants do; they become nests. When it's time for the queen to fatten up and lay a new round of eggs, the worker ants find a nice hollow log, link themselves together like Lego pieces, and form a bivouac, a vast, quivering, climate-controlled haven for the colony's crown jewel. Most bivouacs are the size of a basketball, Dr. Kaspari said, "but I've seen ones as big as a couch."
When the eggs hatch and there are multitudes of young mouths to feed, the workers light out in search of prey. The ants are savage, relentless, capturing as many as 30,000 prey items in a single day. They scale trees to pull down giant scorpions, raid wasps' nests and overwhelm the defenses of even the most violent Africanized bees. Any life form lying in the ants' path knows its only chance is to try to leap away -- and the antbirds know it, too.
The birds swoop down and pick off as many of the harrowed menu items as they can get away with, a kleptoparasitism that ends up reducing the ants' hunting success by one-sixth. Each of the three types of antbird makes its grab from a particular position around the swarm.
As the biggest, most dominant member of the antbird guild and an "obligate" parasite -- one that cannot find food on its own -- the ocellated antbird monopolizes the leading edge and snaps up the meatiest prey. The bicolored antbird, also an obligate parasite, occupies the side position and takes in the second-order invertebrates. Spotted antbirds content themselves with the dregs at the rear. After all, they're only discretionary parasites and can compensate later with independent foraging.
In the new work, the researchers compared the standard three-part scrimmaging of antbirds seen on the Panamanian mainland with the situation on Barro Colorado Island, where the ocellated antbird recently went extinct. They expected that the bicolored antbirds on the island, as the beta birds of the pecking order and obligate parasites besides, would be the biggest beneficiaries of the loss of competition.
Instead, it seems that the spotted antbirds are the ones making the most of the newly opened niche. For one thing, while the island's population of bicoloreds has stagnated and may even be shrinking, the number of spotted antbirds is on the rise. Moreover, some spotted antbirds on the island are clearly losing interest in maintaining and defending specific territories, as spotted antbirds on the mainland vigorously do.
In tricky field experiments just now under way, Dr. Touchton is using checkered flags and other novelty objects to determine whether antbirds that score low in neophobia -- fear of the new -- are the ones most likely to roam. As she explained, obligate parasites can't be tethered to static real estate. If ants are always on the move, their true moochers must follow. Home, for them, must be where the swarm is.
This article originally appeared in The New York Times.