BARRO COLORADO ISLAND, Panama Here in the 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 photo in a dermatology text book. For another, sooner or later – OK, 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, antbirds are almost sure to follow.

The birds are not foolish enough to try to eat army ants, which have fierce mandibles and are 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 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. While 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 Touchton, a researcher associated with the Smithsonian Tropical Research Institute and Princeton University, is the principal author of the report. Now she is 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?

Touchton, 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,” Touchton said. “I want to see it happen.”

Food chain of parasitism

The antbird story also demonstrates the amazing food chain 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 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 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.

Living off savage plunder

All three types of antbirds are adapted to the 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 the approach of the most formidable species of army ant in those parts: Eciton burchelli.

“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 of these encampments are the size of a basketball, 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 and 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 fleeing menu items as they can get away with, a type of plundering 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 species of antbird – an “obligate” parasite 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.

The three-part attack

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, 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, 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.

For those antbirds, home is where the swarm is.