The dinosaurs have been the predominant life form on Earth for the last 250 million years. The rose up on their hind legs in the Jurassic and marched off to rule the world – their skins shimmering with the iridescence of feathered scales. All the other animals of that age retired into hiding from the dominant reptilians.
The reign of these creatures was interrupted three quarters of its way to the present by a cataclysmic mass extinction. All forms of animal life “larger than a bread box” – including all of the “non-avian” dinosaurs – met the end of their evolutionary lineages then. But the most aggressive of the reptile-derived lines, the flying raptors, continued their predominance into modern times.
The limitations of weight on flight have curtailed their maximum size – the dinosaurs are no longer the largest creatures on the planet. But they are still by far the most numerous of the apex predators. Upon the open ocean, millions of sea birds still top the food chains. And the most numerous of the herbivores on land – the insects – have all perfected their camouflage strategies against the avian dinosaurs they know will be looking down upon all of their hiding places.
The insects have become so cryptic that a forest full of them looks empty, even while their calls fill the dusk. Only the lepidopterans, airborne and agile as they are, escape their nemeses. The butterflies share the air with the modern flying dinosaurs.
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Today’s feathered raptors are specialists, and some of them specialize in disaster. They are poised to seize their earth-bound prey whenever the smaller forms are thrown from cover. The birds look down like vultures, waiting to add further misfortune to the plight of the unfortunate.
The Rain Birds. Local disasters take many forms. Excesses of rain bring troubles for the creatures that hide at ground level or below. The rain puddles into pools, saturating the earth. Animals that burrow into or through the soil are drowned out of their concealment. Thrushes, black birds and jays are waiting to pounce when these smaller creatures are forced up for air.
Ponds and streams overflow their banks, and the waterline advances up and over the low ground. Creatures that live hidden under the deadfall on the earth find themselves collected at the advancing margin of a rising tide. The raptors stalk that margin. Crows and gulls gather to take advantage of the plight of the displaced. Wading cranes, herons and egrets flock to these situations – disaster for the lower forms, bonanza for the feathered hunters.
The Fire Birds. Where the land rises and the season dries, fire replaces flood as the disaster that threatens the smaller creatures. The advancing front of a ground fire forces the insects, the small reptiles and mammals, and the smaller birds from cover. And, just as they stalk the advancing margin of the flood in other seasons, the raptors stalk that advancing wall of flames.
Cattle Egrets, falcons and other hawks look through the smoke and wait in the Australian out-back for their prey to flee to them. Ground hornbills and Secretary Birds on the African veldt, caracaras on the Pantanal do the same. These raptors divide the advancing margin of fire, patrolling its edges, its front, the air roiled with rising thermals above it, and the charred track behind. Creatures that flee have little time to regain the safety of cover while on the run.
The archetype fire bird is the Australian Black Kite. It follows the firelines across the brushlands, stooping on any creature that is flushed into the open. Then, after the fire has run its course, the bird has been seen to perpetuate the flames by seizing a burning stick, carrying it away through the sky to a new location, and dropping it into more dry grass to start another fire.
The Ant Birds. The fiery wave that spills across the ground in the most productive of habitats is a living, breathing beast that flows on millions of tiny legs. Hordes of these stinging, biting pests are the scourge of the populations of creatures that flourish in the rain-forest tropics.
The advancing army flushes its prey from every surface. It rustles and sighs as it goes, like a relentless tide rising above the waterline. A particular scent rises from the advancing column, alerting creatures with sensitive noses. But its approach is most noticed by the raucous ruckus of ant birds that flock at the point of the army ants’ advance.
The ant birds are not eating the army ants. They are eating the ants’ prey. They pursue the small animals chased from cover by the invasion that flows through the niches of fallen leaves, down into burrows and up into trees. The birds steal from a stampede of running, crawling, hopping insects, spiders, centipedes, scorpions, skinks and snakes trying to stay ahead of the ground-borne assault.
The flying thieves snatch their prey from just in front of the army’s advance. The birds are specialists, living in obligate association with the raiding ants. They stand aside, briefly taking wing if they find their feet crawling. Then they peer down from the foliage inches away from the action, waiting to pounce.
There are more species of birds exploiting of the waves of army ants than all the other bird species dedicated to the exploitation of all other waves of destruction combined. This is because the army ants in the New World, and the driver ants in the Old, are on the march every day, in every square kilometer of their tropical range. They are not seasonal or sporadic. They are reliable enough to support a dedicated guild of birds that prospers as their parasites.
The hundreds of species of antbirds have diversified into each of the niches that sustains army ants. This includes higher elevations, hilly up-lands vs. flatter lowlands, riversides, forest edges, and second growth jungle. These specialist birds all have different nesting strategies. Some supplement their diets by foraging away from ant raids. But all are alert to the sound of ants on the march, or to the calls of other ant birds who have found a march underway.
The niche these birds exploit has brought them success. They make up less than ten percent of the tropical avifauna species, but their numbers can make up more than a quarter of the birdlife in areas where raiding ants occur.
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At dawn, ant birds peer into the bivouacs of millions of ants linked into living nets, and watch for signs of a raid beginning. The ants will spill beneath the brush on the forest floor, and the antbirds follow along in the overgrowth above. The woodcreepers among them perch on the wider trunks, while other antbirds cling on the thin vertical perches. Horizontal perches accommodate birds like ant tanagers or ant wrens.
Their pecking orders determine which birds sit closest. The bigger ones, like the Ocellated Antbird, command the space above front and center. Smaller species, such as the Bicolored Antbird or the Rufous Collared Antbird take the flanks. Still smaller ones, like the Scale-backed Antbird hang back, waiting to hawk insects that escape the ants by flight. The largest species – Ant Pittas, ground cuckoos – walk through the foliage ahead of the ants. At any time there may be scores of birds, from dozens of species, accompanying a large raid. They call out to their mates and squabble constantly with each other for the most advantageous perches.
The army ants know nothing of the antbirds looking down on their march. These ants are blind and do not feel the presence of the much larger raptors. They live a world of scents and sensations, following and laying trails of pheromones, immersed in the aroma of freshly turned earth. They react to the constant jostling of the other ants around them, and they reflexively bite down on whatever living prey they encounter.
Their behavioral responses are limited by a simple program that organizes hundreds of thousands of individuals into a single multilegged creature that glides across the forest with no central command. The individuals have no idea of what’s happening when the cricket they are holding is lifted off the ground and shaken until they fall off. All they can do then is run faster, until they find their scent trail again.
Nonetheless, the antbirds and the army ants have coevolved and appear to work together. Some of the prey species may be forced back into the jaws of the ants while fleeing from the birds. But the birds have mastered their niche, and their success can rob the ants of most of their harvest. An equilibrium is in place preventing them from taking so much of the ants’ prey that they starve the ants, and then themselves, into extinction.
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After the columns of raiders have passed, far behind the chaos of the advancing front with its mob of birds, the forest settles back down. The commotion fades into the distance, and the vacated foreground lies still. A thin, quiet river of ants runs both ways through the center of the clearing, carrying non-descript fragments of their booty back to the bivouac.
Gradually, the calm air above fills with gliding, flapping butterflies. Airborne, and agile as they are, these butterflies are not concerned with the ants, or the birds. They are free to fill the glades that have been emptied by the earlier disturbance.
These insects make up a particular guild of butterflies that occupies the space over the tracks of army ants. They are of different sizes and colors (some are transparent) but one thing they have in common is their gender – they are all female. Another thing they share is a niche – one of the most specialized in the forest. Like the antbirds, these insects know how to detect and follow ants. But what they really follow are antbirds.
The butterflies make up a mixed flock. Each has a different larval host plant, and different ways of avoiding its own predators. But they all share a common source of nutrients. They alight on the white spots that mark the passage of the ants. Those spots are antbird droppings. Unused bi-products left behind by the avian foragers are useful to them. Nothing is wasted in the forest.
The butterflies are adapted to assimilate compounds excreted by the birds, for use in the production of their own eggs. Other butterflies are restricted in the number of eggs they can produce by the limits of salts and nitrogenous compounds they accumulated during their larval growth. But these butterflies can supplement the nutrients they hatched with and increase their success through what they can glean from behind the army’s advance.
These butterflies are longer-lived than others of the lepidoptera. They spend their lives in pursuit of the most concentrated source of butterfly sustenance to be found. They congregate from miles around to forage at a movable butterfly feast, where the birds from miles around have congregated to accompany the ants. The addition of butterfly eggs to the foliage, and the addition of the antbirds’ eggs to their own hidden nests, draws from the destruction wrought by the army ants to renew further cycles of growth in the forest.
Raptors of Disasters: notes. The birds are the direct descendants of the warm-blooded dinosaurs. The flying dinosaurs reached maximum wingspans of thirty feet in the Cretaceous. The fossil record is incomplete about the smaller forms, which predominate at their level still today. Maximum wingspans now are no greater than ten feet. The smaller forms have diversified into thousands of niches. Many are still raptors, ready to pounce in situations where their prey become exposed. The Black Kites of the Australian out-back have been reported to perpetuate the fires that flush their prey from the tall grass (Montague, 1970). This raises the possibility that once upon a time, at the dawn of human history, these firebirds by their example taught another animal about the handling of fire.
Eciton burchellii, in the American tropics, is the archetypical army ant legionnaire. Its members participate in complex maneuvers organized at the individual level (Franks et al, 1991). Their communication with each other and with their environment is olfactory by pheromone, and tactile by touch. The vision of the soldiers is restricted by an eye that consists of only a single optical facet. That simple vision tells them the difference between night and day and not much more. The operations they launch are diurnal, predictable, and so they are well suited to parasitism by the ant birds (Willis & Oniki, 1978). There are more than a hundred species of army ants; E. burchellii is exceptional in the broad net it casts for prey – most of these driver ants are specific predators on the hives of other ants or social insects.
The butterflies that follow in the wakes of the raids are Ithomiidae, long-winged members of the Monarch subfamily. The guild of Ithomiids described here are understory inhabitants of the tropical forest, who follow the birds that follow the ants (Ray & Andrews, 1980).
Franks, N. R. et al (1991) The blind leading the blind in army ant raid patterns: Testing a model of self-organization. Journal of Insect Behavior 4, 583-607
Montague, A. (1970) A remarkable case of a tool-using bird. American Anthropologist 72, 610
Ray, T. S, & Andrews, C. C. (1980) Ant butterflies: Butterflies that follow army ants to feed on antbird droppings. Science 210, 1147-8
Willis, E. O. & Oniki, Y. (1978) Birds and army ants. Annual Review of Ecology and Systematics 9, 243-63