Antbird Aggregations: The Dynamics of Foraging Formicarids
Note: The following is a write-up I did for the February 1998 Song Sparrow, a publication of Columbus Audubon, as part of a series I authored on tropical ecology. It is based on a term paper I wrote for an animal behavior class at Ohio State. The original term paper ("Dominance Hierarchies in Foraging Antbirds"), which I have since lost, was declared by the professor to be "the best term paper [he'd] ever seen", and he devoted an entire class to reviewing it in detail. He said he deposited it into the archives of the Botany & Zoology Library at Ohio State, but I haven't been there to look it up. That paper contains the original references, most of which were by the late tropical ornithologist E.O. Willis, who devoted much of his career to studying antbirds. Because of the audience and space limitations, citations were not included in the version published in the Song Sparrow.
Tamia añangu is the name the Quichua people of eastern Ecuador use for the army ant Eciton burchellii. In a 1969 paper, E.O. Willis reported finding a specimen of the Hair-crested Antbird tagged with the Quichua name for the species, tamia añangu pishcu, an apparent reference to its ant-following behavior. Some thirty species of antbirds are specialists in this type of behavior. With approximately 230 species, the antbirds (Formicariidae) form the fourth largest family of Neotropical birds. The name is derived from their ant-following behavior, which is exhibited to some extent in nearly all members of the family.
But why, you ask, would a flock of birds want to follow ants? For those of you who think you know the answer, you're probably wrong. Antbirds do not eat ants. Very few species do. Ants produce substantial quantities of formic acid, a substance that is very distasteful to the birds.
The answer lies in a very interesting behavior exhibited by this eusocial insect: the swarm. (Individuals of a eusocial species are willing to commit altruistic acts for the benefit of the group and detriment of self. In most species, this is because the majority of the group's members are related to some degree.) E. burchellii swarms of great size are common in the tropics (as any researcher who has stumbled across one can painfully attest) and are capable of consuming most any small animal in its path. At the head of the swarm is a cloud of hundreds of fleeing arthropods - primarily spiders, roaches, crickets, and katydids - that are flushed toward the front of the swarm in an area about five meters wide and one meter deep. The abundance of potential prey gradually decreases into a wide and resource-poor area toward the rear. The result is that not all areas of the swarm are equally productive so birds compete for advantageous foraging positions along the front of the swarm.
Antbirds are found across mainland Neotropica and on nearby islands primarily in lowland rainforest. Most are of a rather dull black, gray, olive, rufous, or brown plumage and have brightly colored patches of bare skin around their eyes which may serve as "eye spots" to frighten away competitors or potential predators. Some also have brightly colored patches on the wings which are displayed in periods of excitement.
[audio http://www.xeno-canto.org/sounds/uploaded/LHJJCLSCBM/XC277462-GAntshrike.mp3 ]
Most antbirds forage in a similar manner moving from twig to twig in an attempt to keep toward the front of the swarm. Usually, they will perch no higher than one meter above ground and will sally downward after a fleeing prey item or lunge outward from the perch.
Occasional sallies to the ground can be risky activities: in separate papers, E.O. Willis reported witnessing both Spotted Antbirds and Red-throated Ant-Tanagers "hopping violently about" as they were subjected to ant bites or stings.
Although prey size averages close to the length of the bird's bill, Willis once observed a Spotted Antbird carrying an Anolis lizard twice the length of its head a considerable distance before dropping it. The prey is usually beaten against a twig before being swallowed.
It seems quite obvious that antbird flocks are the product of high-level adaptation to the utilization of a single, limited food source. However, several advantages to flocking have been identified (for birds in general) and like serve as a secondary benefit to formicarids: better detection of predators, the "confusion effect" on predators (predators become confused when there are too many options), and mutual aid in finding food. Call notes are important in keeping foraging flocks together and antbird calls may attract other antbirds from the surrounding forest. Competing species are attracted to the recorded call of the Bicolored Antbird, a widespread ant-following species. I once "pished" in a Chestnut-backed Antbird in Costa Rica. The bird quickly fled upon realizing it had been fooled.
The frenzied activity around an ant swarm is a lesson in inter- and intraspecific competition. Commonly, 20 - 26 individuals of one or two species are present, with scattered individuals of as many as thirty other species. Usually, "professional" ant followers maintain optimum positions along the front of the swarm with horizontal spatial displacement of other individuals in the form of "dominance rings" often dictated by the size of individual birds. Such dominance hierarchies based on size are common in many mixed-species flocks, including Ohio's mixed winter woodland flocks of (in decreasing order of size and dominance) Downy Woodpeckers, White-breasted Nuthatches, Tufted Titmice, and Carolina Chickadees.
Members of antbird flocks usually do not tolerate overcrowding and "personal space" is possibly correlated with the size of the individual bird. When another bird violates this spatial separation it is usually greeted with a threat display. Such displays are frequent and may occur within or among species. Many antbirds warn of an imminent attack with displays utilizing brightly-colored wing patches, hisses and grunts, or extension of the head and neck to present the bare facial areas. If the aggressor is not satisfied with the response, an attack will typically ensue and the subordinate bird is displaced with a hiss and snap of the bill.
After being supplanted, most antbirds simply wander a few meters above the swarm and drop into an unoccupied slot. If none is available, they will forage at a higher level or on the periphery - areas which are not nearly as productive. Often, subordinate birds are reduced to "thievery" of arthropods from homeward-bound ants.
The interdependence of army ants and ant-following birds is an interesting topic in need of greater research. It is unlikely that swarming ants would have evolved without the presence of foraging birds to stop the upward escape of prey (one of biology's true "...the chicken or the egg?" questions). The benefit to antbirds is obvious in maximizing foraging efficiency. This benefit is not lost on "accidental" flock members like Nearctic passerine migrants which occasionally join the feast.
Most antbirds will not leave the sanctuary of the forest, even to cross a road or stream. This helped to isolate populations (both geographically and genetically) and possibly served as a partial mechanism for their great diversity. In the age of backhoes and chainsaws, fires and forest fragmentation, what chance do such forest-dependent species have?