For North American birders, the term “bird spectacle” connotes numbers--spectacular numbers—thousands of Broad-winged Hawks migrating over Hawk Ridge in Minnesota in September, thousands of Snow Geese lifting off simultaneously minutes before sunrise at the Bosque in New Mexico in November, thousands of Northern Gannets nesting side by side on St. Bonaventure Island in Quebec in July. Largely because of habitat and terrain, however, bird spectacles in the tropics are much more about quality than quantity, and one of our most anticipated Costa Rican rain forest spectacles was an attended ant swarm. Our first one last month did not disappoint.
Like roadrunners in Arizona, ant swarms in the tropics are unpredictable, and even finding one does not guarantee good birds or any birds. We encountered out first ant swarm at La Selva in 2009, six feet wide running perpendicularly across a six foot trail and extending who knows how far into the forest on either side, a literal river of ants but, sadly, unattended by any birds at all. It served only to heighten our anticipation of someday seeing the suite of small, generally dull colored family of birds, known as antbirds, which live in the deep shadows of the forest and sometimes follow the ant swarms, not to eat the ants, but to harvest the bugs which the swarms flush.
Army ants are incessantly on the move, never establishing a permanent nest, but simply bivouacking at night forming, in a sense, a living nest with the queen, the larvae, and the eggs in the protected center. Each morning they swarm out to a different compass point, devouring luckless insects and arthropods hiding under the leaf litter. Swarms may contain thousands of individuals, cover a swath twenty yards wide, and consume 100,000 prey items per day. Those are large numbers, but the spectacle is the antbirds which follow them, devouring the bugs desperately trying to escape the swarm. The most spectacular are the Ocellated Antbird, eight inches of rufous with black scallops and a large patch of bare blue skin around the eye, and the Immaculate Antbird, seven inches of uniform black plumage plus the blue orbital patch.
We came across our attended ant swarm last month near Arenal, oblivious at first to what we had walked into, partly because the ants were crossing the trail diagonally and swarming along the forest edge, partly because the attendees were largely silent and the only sound was the occasional thwack of an antbird bill closing on a moth, cricket, or cockroach. We saw the antbirds—Immaculate, Bicolored, Ocellated, and Spotted-- before it dawned on us why these four Lifers we had never before seen were all here in the same place. Then we saw the ants and realized we were experiencing a bird spectacle, tropical version.
When a non-birder first hears the term “antbird,” the assumption is “birds that eat ants.” A friend to whom I explained the spectacle called it a “well oiled machine,” an astute observation perfectly describing how nature binds different species together for the benefit of all. Another layer is added to the top of this food chain when the swarm is sometimes also attended by forest-falcons, a family of tropical raptors that hunt much like our accipters and pick off the antbirds accompanying the ants. The biomass of the tropics is staggering in its immensity and complexity. That, in and of itself, is the bird spectacle of the tropics, quality over quantity.