Related Resources
Spotlight on Bird Aggregations from the Zoo's Migratory Bird Center
Research on Shade-grown Coffee from the Zoo's Migratory Bird Center
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Birds in Swarm's Way
by Howard Youth
"Whoa! Look over there by the edge of the road," whispered my wife, Marta, her binoculars on the rise. A kiskadee flycatcher, its belly bright as a lemon and its wings rust-orange, flapped from the ground to the trees with a large gray grasshopper dangling from its bill. "Over on that white post," I blurted. A plush-crested jay took center stage, its shaggy head cocked, its blue-ringed yellow eye scanning the ground. In a flash, it hopped down to snap up another grasshopper. A pair of large squirrel cuckoos appeared next, with flowing black-and-white tails, and, on either side of them, four disheveled smooth-billed anis scrabbled, intent on cleaning up at the invertebrate brunch. Once we identified these striking birds, we noticed less flamboyant ones, including pale-breasted and rufous-bellied thrushes and a pair of fawn-breasted tanagers.
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| Toco toucans, which sometimes nest in termite mounds that have been excavated by woodpeckers, are among the birds that benefit from the industry of colony insects. Other bird species follow army ant swarms, preying on animals that the ants flush into the open. |
We couldn't believe our luck. We knew that a visit to Iguazú National Park at the subtropical border between Argentina and Brazil would give us a good chance of seeing interesting birds. But we never expected to see so many in one place. Then we discovered the reason: ants. Black legions of them coursed through the grass at the forest edge, sending a smorgasbord of larger insects scurrying right into the bills of the birds. We learned that, when birding in or near dark, deceptively quiet rainforest, it's best to listen for the skittering of panicked prey and the chatter of excited birds that mark an army ant swarm.
Feathered creatures have shared the planet with ants and other colonial insects for millions of years. Back in the Cretaceous Period, between 144 and 65 million years ago, when primitive flying birds shared the skies with winged reptiles called pterosaurs, and tyrannosaurs stomped across steamy landscapes, ants, termites, bees, and wasps emerged as a powerful force. Today, they account for an estimated 75 percent of the planet's insect biomass. This summer, while people across the country wield swatters, hoses, and spray cans against them and shout "What good are they?" it's clear that, away from human dwellings, these insects are extremely important within their habitats and to their neighbors. Made of tiny constituents that are together mighty, an insect colony can be a nuisance or danger, or for birds, an essential resource to be followed for the great opportunities it offers.
"Ants are everywhere, but only occasionally noticed," wrote entomologists E.O. Wilson and Bert Hölldobler in their 1990 classic The Ants. "They run much of the terrestrial world as the premier soil turners, channelers of energy, dominatrices of the insect fauna—yet receive only passing mention in textbooks on ecology." But step off a forest trail in the tropics, and you can't help but notice these omnipresent, industrious insects. In Amazonia's terra firme forests, ants and termites alone account for an estimated one-third of the entire animal biomass. They are likely just as abundant in many other habitats across the globe, according to Wilson and Hölldobler.
The most feared and celebrated are the army ants. According to Wilson and Hölldobler, about 300 of the world's approximately 9,000 ant species can be called "army ants" for their frequent migrations and group-hunting habits. Most army ants live in the tropics and subtropics, including southern Mexico, Central and South America, Australia, Africa, and Asia. But a few species reach as far north as Iowa and Virginia.
Eciton burchelli is the best-known army ant in Central and South America. A colony of these large, long-legged ants ranging through the forest does not have a fixed address. Wherever the colony settles for the night, usually at a sheltered site on the forest floor, is its home. Several castes of ants exist within the colony, including hundreds of thousands of workers. At night, workers link legs and form layer upon layer of ant chain, protecting one elongated queen and many immature ants at its center. Several virgin queens and males—the raw materials of future colonies—are also in attendance for a short time each year. All told, the massed and resting colony, a ball called a bivouac, can span three feet in diameter.
As morning light filters to the forest floor, the ball unravels and the ants head out on their daily hunt. Waves of workers push forward, laying down chemical markers, or scent trails, from the tips of their orange abdomens to guide compatriots that follow and soon take the lead. An E. burchelli swarm rolls across the forest floor like a line of miniature bulldozers, plowing over, killing, and breaking apart invertebrates such as tarantulas, scorpions, roaches, beetles, grasshoppers, and ants of other species, as well as small snakes, lizards, frogs, and other small creatures that don't make it out of the way. Bits of prey are transported back to the colony's current bivouac site, where the queen and young are tended by workers. Meanwhile, larger, bulky-headed soldiers cruise the shoulders of the ant highway, defending workers as needed.
Army ants are top predators of many forest floor invertebrates, but pose little threat to most mammals and birds. The dean of Neotropical ornithology, Alexander F. Skutch, chronicled Central American bird behavior for more than 70 years, until his death in 2004. Many times he watched army ants course over and around incubating birds, but he wrote that "I have never known them to harm a bird of any kind." Skutch considered army ants far less of a threat to birds' eggs and broods than other types, such as fire ants (Solenopsis spp.). Apparently, movement spurs army ants to attack, and incubating birds and some large insects escape notice by sitting stock still.
So, for birds, army ant swarms are not usually a cause for panic, but instead offer a great chance to snap up a wave of fine living foods. In Amazonia, up to 50 bird species may be represented in a single army ant-following flock. These include obligate species, which always attend swarms and appear to be dependent upon them, as well as opportunists that make the most of it when they happen upon a swarm. Swainson's thrushes (Catharus ustulatus) and rose-breasted grosbeaks (Pheucticus ludovicianus) are among the Neotropical migrants—birds that nest in North America but winter in the tropics—that opportunistically join frenzied resident antbirds, antthrushes, antpittas, antshrikes, tanagers, and woodcreepers at army ant swarms. Resident birds, however, often chase these migratory birds away, leaving them to snatch whatever scraps they can at the edge of the swarm. Larger Central and South American birds known to follow ant swarms include motmots, some toucans, and the 19-inch-long rufous-vented ground-cuckoo (Neomorphus geoffroyi). As they follow army ants, these birds attract butterflies that feed on their nutrient-rich droppings.
Hunting and bivouacking habits vary among army ant species and genera. Driver ants (Dorylus spp.) are the most infamous army ants of Africa. Their colonies tend to contain far more workers than those of E. burchelli, and they often cluster below ground to excavate their nests. Wilson and Hölldobler wrote that Africa's "driver ants are not really the terror of the jungle as popularly conceived. Although the colony is an ‘animal' weighing in excess of 20 kilograms [44 pounds] and possessing on the order of 20 million mouths, its raiders move over the ground at the rate of only a meter [just over three feet] every three minutes. It is possible to watch the whole process at close range while seated comfortably in a camp chair… ."
As driver ant columns move across African landscapes, they resemble thick black ropes draped over the ground. In sub-Saharan undergrowth, a sinuous colony of millions may, among other creatures, attract opportunistic songbirds such as red-tailed antthrushes (Neocossyphus rufus) and white-starred robins (Pogonocichla stellata). The antthrushes feed on the ants and the insects they flush, while the robins favor just the fleeing insects. In central African forest, the chocolate-backed kingfisher (Halcyon badia), which normally sits high in the trees looking for prey, perches low when following driver ant columns to gobble up some ants along with the grasshoppers, beetles, mantises, crickets, and other prey the ants kick up. Like the antthrush and kingfisher, many woodpeckers eat ants. But most other birds avoid them, probably because the formic acid exuded by ants renders them distasteful. That same formic acid, however, likely makes them desirable as a preening accoutrement. (See "Ants, a Quick Picker-upper.")
Many aspects of army ant ecology remain unstudied, but a consensus is emerging among scientists that the ants are important to a host of animals that would not flourish without them. "Their triple importance as predators, prey, and hosts for parasites and commensalists strongly suggests swarm-raiding army ants have keystone functions in tropical forest communities," wrote biologists Caspar Schoning, Wanja Kinuthia, and Jacobus J. Boomsma in the Journal of East African Natural History in 2006.
But at least some of these formidable insects have an Achilles' heel. In 2000, biologists Dina L. Roberts, Robert J. Cooper, and Lisa J. Petit, from the University of Georgia and the Zoo's Smithsonian Migratory Bird Center, reported in the journal Conservation Biology that two widespread Neotropical army ants, E. burchelli and Labidus praedator, thrive in Panamanian natural forest and shade-coffee plantations but are absent in the country's exposed sun-coffee plantations. Among the likely reasons: The ants, which hunt in thick leaf litter and, in the case of E. burchelli, seek woody materials such as fallen logs for bivouac sites, can't find enough of this matter in the warmer, drier sun-coffee plantations.
In another study published in Ecological Applications in 2000, Roberts, Cooper, and Petit tallied ant-following bird species in Panamanian mid-elevation forest, adjacent shade-coffee plantations, isolated shade plantations, and sun-coffee plantations. All habitats except sun-coffee plantations supported large numbers of ant swarms, and the team tallied a total of 126 ant-attending bird species. But forest and shade-coffee plantations adjacent to forest held the most intact bird assemblages when compared with isolated shade-coffee patches. The authors hypothesized that "army ants may…be a mechanism for bringing forest birds into shade coffee, but only to a point."
Disappearance of shaded forest habitat, and the subsequent disappearance of army ants, does not bode well for birds adapted to finding food flushed by ants. In the Neotropics, the most vulnerable are the obligate ant followers, such as the white-plumed antbird (Pithys albifrons), a fixture at many Amazonian ant swarms, and, across most of the range of E. burchelli from Mexico to Brazil's southern border, the ruddy woodcreeper (Dendrocincla homochroa) and gray-headed tanager (Eucometis penicillata), among others.
The effects of forest loss on army ants appear to be very different in East Africa, at least for driver ants. In their 2006 study, Schoning, Kinuthia, and Boomsma surveyed Kenyan forest plots of varied sizes as well as adjacent open habitats, and concluded that populations of driver ants did not seem to suffer from forest fragmentation: They seem to do just as well in farmland and other sunny, disturbed landscapes as they do in shady forest.
Other colonial insects attract birds and ants. Among them are termites, which are eaten by ants, birds, mammals, and reptiles. There's quite a menu from which to choose: Worldwide, there are more than 2,700 termite species. Most of these feed on dead wood and other plant matter, and fewer than 200 colonize houses.
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| Termites build nests that are sometimes taken over by birds. (Scott Bauer/ARS) |
Craggy termite nests, or termitaria, are familiar adornments to many tropical landscapes. Termites may nest below ground, on the ground, or in trees, depending upon their species and habitat. Their termitaria are often large and riddled with chambers that regulate the temperatures within, keeping the colony from baking in the tropical heat that blazes just outside. Inside, the cool, dark interior shelters a long-lived queen (she may live for several decades) that lays all the eggs, while sterile workers labor to maintain and build the nest and its chambers.
Just as prairie dog colonies shelter snakes and ferrets, termitaria attract their fair share of freeloaders. Birds in the colorful trogon family, for example, favor arboreal termitaria as sites for their tunnel nests. All trogons are cavity-nesters, and almost a third of New World species will sometimes or often excavate their nests in arboreal termitaria. At least one, the white-tailed trogon (Trogon viridis), nests virtually nowhere else.
In a study of Peruvian termitary-nesting birds published in the journal The Condor in 2000, biologist Donald J. Brightsmith reported that trogons and some other birds seem to prefer to nest in termitaria also inhabited by aggressive, biting ants (Dolichoderus spp.). These ants may unwittingly protect the birds' nests by discouraging predators or by providing "olfactory camouflage" with their strong scent that masks nest odors, and thus keeps the birds safe from night-prowling rodents and marsupials. There is anecdotal evidence that the ants also help tidy the birds' nests, ridding them of feces and ectoparasites.
Taking advantage of their termite hosts' remodeling skills, a trogon pair take turns gouging out their nest chamber. They typically choose active termitaria, which are sturdy enough to withstand excavation. Eating termites as they work, the trogons keep the cavity and chambers open while the insects busily repair interior walls. When the trogons abandon their nest after their chicks have fledged, the termites reclaim what's rightfully theirs, patching up all the walls and resealing the excavated cavity.
The violaceous trogon (T. violaceus), which skulks at forest edges from Mexico to Brazil, ventures where few others will. In his field guide to Costa Rican birds, Skutch wrote that this trogon nests not only in termitaria but also "in a chamber carved in a high gray, top-shaped arboreal nest of Parachartergus wasps, less often in a blackish, stalactite-like, arboreal nest of Azteca ants, in a decaying trunk, or in a dense mass of roots of a large fern or other epiphyte." The violaceous trogon is the only New World bird known to nest in ant and wasp nests. (In Africa and Asia, a few barbets and woodpeckers nest in arboreal ant nests.) Before taking up residence in a wasp nest, a trogon pair ensure that their young won't be stung: They systematically kill all of the resident wasps.
While they may avoid wasp and ant nests, birds other than trogons nest in termitaria. Dagger-billed Neotropical aerialists called jacamars burrow into arboreal termitaria with their bills and kick debris away with their small feet. Chunky, thick-billed puffbirds also chip away at and nest in termitaria, as do about two dozen parrot species—roughly ten percent of the world's 250 species.
Not all termitaria are created equal in the eyes of parrots and some other birds. A natural partitioning seems to occur, based on what each species considers the optimal nest site. In the Peruvian Amazon, for example, Brightsmith found no shortage of termitaria but noted that black-tailed trogons (T. melanurus) used larger termite nests situated lower in trees than those used by cobalt-winged and Tui parakeets (Brotogeris cyanoptera and B. sanctithomae). While the parakeets chose similar termitaria, the cobalt-winged parakeets nested in mature forest and the Tui parakeets nested in edge and young forests.
Towering ground termitaria also provide a variety of birds with food, look-out and territorial perches, and nesting opportunities. In Africa, for example, barbets in the genus Trachyphonus burrow into earth banks or terrestrial termitaria, tunneling up to 16 inches into a mound. In South America, even tree-nesting toco toucans (Ramphastos toco) sometimes occupy burrows previously chiseled into terrestrial termitaria by nesting woodpeckers called campo flickers (Colaptes campestris).
Combing the Forests for Honey
In Africa and a few parts of Asia, frumpy-looking birds called honeyguides frequent beehives to feed on the bees as well as their wax, larvae, and eggs. Most of the world's 17 honeyguide species consume beeswax, but none is large or strong enough to tear open a hive. For that, they rely upon mammals such as humans and African honey badgers (Mellivora capensis) that seek the honey inside the hives.
Most honeyguides follow these hive-opening mammals, but the greater honeyguide (Indicator indicator) actually takes the lead, actively drawing people to wild beehives. The cardinal-sized bird's persistent trills, which sound like a shaking match box, provide the first clue that honey is ahead. Then the bird takes off and shows the way to the hive with a series of short, undulating flights, its bold white outer tail feathers flashing. When it reaches its target, the honeyguide quiets down and perches nearby, then waits for the hive to be found and opened, its waxy treasure exposed, and its stinging residents evicted. Guiding is likely a learned behavior: In suburbs, such as those around Nairobi, honeyguides no longer attempt to guide, apparently because links between mammalian hunters, birds, and hives have broken down.
Much remains to be learned about the intriguing ties between invertebrates and birds. Their complex interactions provide fodder for future scientific study and cherished memories for nature lovers lucky enough to glimpse them in action. It's been a decade since Marta and I watched the Iguazú bird feeding frenzy, but those few sublime moments forever cemented in our minds the importance of the links between beings avian and invertebrate.
—Contributing editor Howard Youth last wrote about lekking prairie grouse in the March/April 2007 issue of ZooGoer.
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ZooGoer 36(4) 2007. Copyright 2007 Friends of the National Zoo.
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