Cave-Dwelling Birds

True creature of night is the guacharo, or “oil bird”, of northern South America. It is reddish-brown, about the size of a barnyard hen. Excessive layers of fat built up about its abdomen formerly were valued highly by natives for eating purposes, resulting in the slaughter of countless thousands every year. The guacharo spends its days a half mile or more deep in the interior of mountain caves. Here it roosts and builds its nests in crevices high in the rock walls. It leaves in groups of twenty to thirty shortly after dusk and apparently spends the whole night foraging for food, sometimes covering as much as 200 miles.

Like the cave bat, it seems to have no difficulty finding its way in absolute darkness. An explanation of this ability, acoustic orientation, has been reported by Dr. Donald R. Griffin of Cornell University. The birds apparently are guided by echos of specific sharp “clicking” sounds which they make.

“The individual click,” Dr. Griffin explains, “consists of a very few sound waves having a frequency of about 7,000 cycles per second. The duration of each click is about a millisecond (1,000th of a second). The clicks were loud enough to be audible easily about 200 yards inside the cave. Except for their lower frequency, these sounds are very similar to those used by insectivorous bats for their acoustic orientation.

“The external ear canals of three captive birds were plugged with cotton. They then became disoriented when flying in the dark. They collided with every object they encountered. Before and immediately after this treatment they flew about in a small dark room avoiding all collisions with the walls.”

Their best known habitat is the guacharo cave in Venezuela’s Humboldt National Park, where they are rigidly protected. Most of them nest in a vast subterranean hall more than a half mile long and a hundred feet high. Here more than a thousand of the birds greet the intruder instantly with a wave of awesome and deafening shrieks.

“With the advent of dusk,” reports Dr. Eugenio de Bellard Pietri—Venezuelan cave explorer, “the birds come out in compact groups but before the exodus a preliminary flight is held by a few as if to make sure that night is falling. Soon they return to the depths of their somber mansion, evidently to give the flock the all clear signal. Late in the evening there is not a single adult specimen left in the cave. The flight of these birds is silent and cannot easily be detected.”