Army Ant Raid — Osa Peninsula Costa Rica
Army ant raids in Costa Rica's Osa Peninsula are one of the most violent and frenetic spectacles in tropical biology — columns of 200,000 Eciton burchellii ants sweeping through the forest floor in a 15-metre-wide front, flushing every invertebrate, lizard, and small mammal from the leaf litter while 25 species of antbird hover in the advancing column to catch fleeing prey. The ants are entirely blind and navigate by pheromone; the birds — including ocellated antbirds and bicolored antbirds that follow no other food source — are entirely dependent on the ants to flush their prey. The front advances at roughly 20 metres per hour, and standing at the leading edge as the forest floor literally comes alive with fleeing insects and birds arrive from every direction is one of the most intense sensory experiences in tropical rainforest. Raids occur almost daily in dry season colonies.
About this spectacle
Standing at the leading edge of an army ant raid in Corcovado National Park is one of tropical biology's most overwhelming sensory experiences. A 15-metre-wide front of up to 200,000 Eciton burchellii sweeps through the leaf litter, entirely blind, guided only by chemical trails, and the forest floor writhes as every beetle, cockroach, spider, lizard, and fleeing invertebrate tries to escape. The air around the column fills with antbirds — ocellated antbirds, bicolored antbirds, and up to 25 species total — darting low to snatch prey flushed by the advancing mass. The sound is a combined rustle of leaf litter, insect wingbeats, and bird calls converging from every direction. The column advances at roughly 20 metres per hour, so the spectacle plays out slowly enough to watch and photograph, yet the density and urgency of activity is relentless. Raids occur almost daily in dry season, typically peaking in morning hours, deep in Corcovado's primary rainforest.
When to go
Jan — Dec, peak Dec — Mar
Getting there
Nearest airport: PJM. Nearest city: Puerto Jiménez.
Booking options
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