Movers & shakers and kleptoparasites
I was inside the engine of the tropical rainforest. It was a drippy July morning at the Arenal Observatory Lodge reserve near La Fortuna, Costa Rica. Alone on the forest trail, staying quiet and alert as army ants marched and foraged parallel to the trail, I was accepted by three species of “obligate” antbirds, also known as kleptoparasites, that permitted me to observe their behavior up close. They glanced my way from time to time, but then went back to work, scanning the moving army from a foot or two above ground, jumping down to gulp insects in an instant and then returning to a perch above.
The Bicolored, Spotted and Ocellated Antbirds survive primarily by following the literal movers and shakers of the forest floor. I also saw a Buff-rumped Warbler nearby, but not in the middle of the swarm. They too take advantage of ants, but they are more diverse in their territories.
Eciton burchellii are almost certainly the species of army ant I observed that day, as it is the most predominant species in Costa Rica. There may have been a half million ants moving through as I watched from the trail for one and a quarter hours. I calculated whether I could suddenly become surrounded and determined that I could make a run for it if necessary.
They kept coming in a swath that looked two meters wide. The antbirds (also included in that group are thrushes, wrens, ovenbirds, antshrikes, woodcreepers, tanagers, and others) in an E. burchellii entourage in wet tropical forests depend on the ant predators to flush out insects, other arthropods and spiders from decomposing leaves and other detritus. Antbirds do not eat ants. Carl and Marian Rettenmeyer* spent their professional careers determining that 550 diverse species of animals co-exist with army ants (including mites that cling to their feet), and around 300 species depend on army ants for their very survival.
Since I first encountered army ants and a bird entourage in Corcovado National Park in 1999, I have wanted to see this phenomenon again and have a camera ready to capture the bird activity. Those 75 minutes were thrilling for me; I will never forget the sense of being part of their world for a time. A sudden appearance of Coatimundis took my eyes off the birds and ants. About forty or more family members emerged silently from the forest, most crossing the trail into the forest on the other side. A couple of juvenile coatis diverged, coming over to smell my shoes. They sniffed, looked me in the eyes, and then went quietly on their way.
Army-ant colonies require more than 10 ha to maintain their worker force**. At Finca Cantaros, my 7-ha former property in the Zona Sur of Costa Rica, there were just leaf-cutter ants, and lots of them.
*Ed Yong, An entire world follows the march of the army ants, National Geographic, 11/30/10.
**E.O. Wilson, The Diversity of Life, Harvard University Press, 2010.