Spotted Antbird attending a swarm of Army Ants- Soberania National Park, Panama
We carefully picked our way along a muddy trail in the rain forest at Soberania National Park in Panama as we tried to take in all the unfamiliar sounds coming from the jungle’s birds, insects and frogs. On this day there were five of us, four birders following closely behind Jorge, our guide, A few of the songs coming from the dark forest were recognizable to me- the descending double notes of the Spotted Antbird and the maniacal wheezing of a Bicolored Antbird. Suddenly Jorge signals us to stop- he says in a hushed tone- “Hay hormigas”- there are ants.
Birding in a tropical rainforest can be a humbling experience. Its easy for a first-time (or tenth time) visitor to the rainforest to be overwhelmed. Overwhelmed by the heat, the humidity, the sights, the sounds and especially, the insects. Crawly things, large and small, are everywhere, and although they seldom have any direct impact on the human observer, they are pretty hard to miss. One thing that gets the birder to snap to attention though are those whispered words. Ant swarms bring special birds.
While the noise of the cicadas and the flash of the butterflies grab our attention, it’s easier to miss the hordes of ants- the leaf-cutter ants and the army ants- at our feet. surely the leaf-cutters are more interesting to look at, but from the perspective of a birder the army ants are the ones we want to see, and these are the ants that Jorge found.
White-bellied Antbird in the Darien National Park, Panama
Not that the leaf-cutters aren’t fascinating in their own right. They are everywhere in tropical Central and South America. Not just in the deep forests, they also can be found marching through parking lots, along sidewalks and on walls, pipelines, wires- anyplace that can serve as a conduit between their nests and their leafy quarries.
As you have probably seen on many a David Attenborough special, leaf-cutters travel along well defined lines out of the nest and into the vegetation, where the chew off chunks of leaves, then carry them more or less on their heads back to their underground chambers. There, the leaves are used as fertilizer for certain specialized species of fungi, and its the fungi that are served up as food for the ant larvae. The adult ants generally feed on leaf sap.
Leafcutter Ants carrying plant material between underground tunnels
Each species of ant cultivates a special species of fungus, and fungus and ant communicate with each other through a web of chemicals. The relationship is an example of mutualism- the fungus only lives within the ant colony and the ant colony only survives by utilizing that species of fungus.
The leaf-cutter nest contains millions of individuals, divided into at least four separate worker castes, including soldiers (Majors), foragers (Mediae), defenders/ patrol (Minors) and workers who tend the nest and garden (Minims). Minims can sometimes be found riding along on the foragers- perched on top of their leaves or even on their heads- to ward off parasitic flies.
Leafcutters using a fallen log as the path of least resistance. Notice the guards riding shotgun on some leaves to ward of parasitic Phorid flies.
Leaf-cutters haul the debris of their subterranean farming activities to the surface, and this allows the observer to located the nest site.
Fascinating as the leaf-cutters are, it’s the army ant swarms that draw the birds and the birders. Unlike their agrarian cousins, army ants are nomadic carnivores, blindly surging through the forest in search of any prey incapable of escape. Other insects large and small, lizards, snakes, and nestling birds might fall prey to the swarm. Nearly all army ant species conduct their raids underground and at night. But the one we seek out is the exception- Eciton burchelli- diurnal and surface dwelling, it is the one that drives prey into the mouths of waiting birds.
Because they are nomadic, these army ants generally don’t build nests. They form up into overnight “bivouacs” made of their own bodies, packed together and tucked away in the hollow of a tree. There they live out one part of their life cycle- the queen lays tens of thousands of eggs while the workers march through the surrounding forest, killing prey and transporting it back to the bivouac.
The dark areas in this tree crevice were packed with army ant bodies bivouaced for the night. A few feet away, a Rufous-vented Ground Cuckoo roosted to wait for the morning deployment.
After about three weeks, the bivouac disbands and the ants move through the forest on “exploration” mode, spending another three weeks seeking out a new target rich environment in which to establish a new bivouac- rinse and repeat.
Bicolored Antbird “cleaning up” alongside a swarm of Eciton burchelli army ants
Open any bird guide to the American tropics and you’ll see page after page of antbirds, ant-thrushes, ant-wrens, ant-shrikes, ant-pittas and so on. Most of these birds will take advantage of an ant swarm, a behavior which has evolved over a span of several million years.. In fact, some seem to occur only in conjunction with army ant colonies. In addition to the birds with “ant” in their name, there are many other species that show the same behaviors.
Black-crowned Antshrikes may at times join in with flocks of ant swarm followers, but tend to stay on the peripheries, foraging with members of their own family groups.
Gray-headed Tanagers are almost always associated with antswarms- finding one means that an ant swarm is nearby.
Many species of woodcreepers and tanagers, including Gray-headed Tanagers follow ant swarms faithfully, while some motmots, jacamars, puffbirds and others are more casual devotees.
Red-throated Ant-tanager
Red-crowned Ant-tanager
Cocoa Woodcreeper grabbing a cicada
Barred Antshrikes- a female here- often follow ant swarms, but can also be found foraging on their own.
Rufous-vented Ground Cuckoo- a roadrunner sized bird- maybe not much to look at, but definitely one of the “Holy Grails” of world birding. They occur in very low densities, almost always at army ant swarms, and even then they are incredibly hard to see.
Rufous-vented Ground Cuckoo getting tucked in for the night near an ant bivouac.
The very unobtrusive Olivaceuos Flatbill watching as the ants go marching by.
Black-cheeked Ant-Tanager is an occasional follower of ant swarms. This is a Costa Rican endemic- it occurs only in the area of the Osa Peninsula in southwestern Costa Rica.
As you may have guessed, having a large number of birds concentrated into a relatively small area attracts the attention “birders” with more nefarious agendas. There are reports of small, forest dwelling hawks like this Semiplumbeous Hawk following and feeding on the ant followers.
Semiplumbeous Hawks have been reported to prey upon antbirds as they follow swarms.
In my own experience with army ants and leaf-cutters, they seem to avoid interacting with each other, although there is apparently one species of army ant that raids the nest of leaf-cutters, generally causing great devastation on both sides. Generally, though, one group will pause as the other passes. In this video I took in Panama, army ants use a twig as an overpass, while leaf-cutters pass below.
Since these army ant swarms are nomadic, you may be wondering how obligate ant-followers can maintain a breeding territory and keep in touch with their food source at the same time. The answers aren’t well known- there is still a lot of research to be done regarding the breeding habits of these fascinating hunters. It does appear to be the case, though, that at least some species of antbirds maintain a large enough breeding territory that there are multiple ant swarms present within the territory at any one time, so if an ant colony moves on or enters its three week period of inactivity, the antbirds can switch over to follow a different swarm. Research indicates that in certain species, the breeding territory of one pair can be used by neighboring pairs to forage, although the “home” pair generally are shown deference in selecting foraging sites. In the case of Ocellated Antbirds, those neighboring “guests” tend to be offspring of the male territory holder, while in Bicolored Antbirds the neighbors are not necessarily related.
Ocellated Antbird- one of the tougher antbirds to find in Central America. This one was at a large swarm in the San Francisco Reserve in eastern Panama.
The strategy of following along as another species dislodges food items is not unique to ants and antbirds. Think about cows and their attending Cattle Egrets. There are species of tropical hawks who follow troops of monkeys around. The cavorting monkeys dislodge lizards and large insects, which birds like the Bicolored Hawk prey upon.
Bicolored Hawks follow troops of monkeys through the forest- picking up meals as the monkeys dislodge insects and small reptiles.
The behaviors exhibited by ant following species is fascinating and has taken millions of years to evolve to the point where they are now. It’s a feeding strategy that can pay high dividends for relatively low cost- its almost like having food delivered into your home! But, in modern times, there are risks as well. Insect populations are crashing in many parts of the world, and to rely on one species of ant as heavily as some of these birds do could prove to be a dangerous wager. These ants themselves rely upon healthy forests and healthy insect populations for their survival, and as the ants go, so go the antbirds.
