Home's where the ants are

Animal Behaviour

Antbirds in the Panamanian rain forest are clever enough to ensure that their stomachs are full without bothering to hunt for anything on their own. They depend on predatory ants which do all the hard work on their behalf, writes Natalie Angier.

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.

If you don’t tuck in your pant legs to protect against chiggers and ticks, you will end up a colour plate in ‘Rook’s Textbook of Dermatology’. Sooner or later, you will 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 cannot eat army ants since they are fiercely mandibled and militantly cohesive. Instead, they hope to skim off a percentage of the ants’ labour 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. However, here the big vertebrates are the parasites.

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 of not only 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 University, 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. She is collaborating on the project with Martin Wikelski of the Max Planck Institute for Ornithology. In Touchton’s view, studying spotted antbirds offers an extraordinary opportunity to catch evolution on the wing.

“The antbird story also demonstrates the vividly baklavaian nature of parasitism in a tropical rain forest. Researchers have also identified three species of butterflies that specialise in following antbirds. These butterflies feed on bird droppings, and though guano is a notoriously unpredictable resource in a rain forest, they know where their suppliers are likely to be found.

Defensive antbirds

Antbirds belong to a large, old and a tropical avian family of some 200 species. The new research looked at three swarm-stalking species that live in the same region of Panama: the spotted antbird, slightly larger, bicoloured antbird and a larger ocellated antbird.

All three adhere to the basic script of tropical birds, living longer than the two or three years of the average chickadee or sparrow up north. But they brood significantly smaller clutches per year, and males and females sing jointly to prove to the world that they are a formidable team.

All three types of antbirds are adapted to the clotted, shadowy conditions of the understory and they flit more than they fly. These birds have features designed for the ant-following: 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.

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.

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. The birds swoop down and pick off as many of the harrowed menu items as they can get away with, thereby reducing the ants’ hunting success by one-sixth. These birds make their grab from a particular position around the swarm.

The biggest, most dominant member of the antbird guild and an obligate parasite, the ocellated antbird monopolises the leading edge and snaps up the meatiest prey. The bicoloured 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.

Spotted birds on the rise

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 bicoloured 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 bicoloureds 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 under way, Touchton is using checkered flags and other novelty objects to determine whether antbirds that score low in neophobia, 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.