Four Hundred Thousand Bats and the Question Nobody Has Answered

Every evening from late spring through early fall, something happens at Carlsbad Caverns that has no good equivalent anywhere else in the American Southwest. Beginning roughly an hour before sunset, the park's colony of Brazilian free-tailed bats emerges from the natural entrance to the cave. The column starts slowly, a few dozen animals circling the opening, and then accelerates into a sustained spiral of hundreds of thousands of bats that can take two hours to fully exit. Estimates put the colony at approximately four hundred thousand animals, though counts have varied considerably over the decades and the precise number in any given year is not established in the public record.

The bats spend the night hunting insects across the Chihuahuan Desert, each one consuming several thousand insects in a single night, a biological pest-control operation operating at a scale that agricultural researchers have spent decades trying to quantify. They return before dawn, spiraling back into the cave entrance in a diving column that runs in the opposite direction from the emergence.

The colony was much larger in the early twentieth century. Guano miners working the cave between nineteen hundred and nineteen twenty-three extracted approximately one hundred thousand tons of bat guano for use as fertilizer, a figure that offers one measure of how large the historical population must have been. What the miners took out, in biological terms, was centuries of accumulated nitrogen. When the mining ended and the cave was converted to a tourist attraction, the bat population had already been substantially reduced from its pre-contact baseline.

The research that would have established a cleaner population baseline was largely not conducted until the nineteen eighties, and some of that research record is incomplete. What happened to the colony in the decades when nobody was counting is, largely, not in the record.

The evening flight is free to watch from the amphitheater above the natural entrance. It is one of the stranger and more extraordinary things this landscape offers, and it operates entirely on its own schedule.