A Colony of Rare Free-Tailed Bats

Abstract

A small crevice in a sheer cliff at 6200 feet elevation in the Chisos Mountains of Brewster County, Texas, houses the only colony of Tadarida macrotis (Gray) thus far discovered in the United States. The Chisos Mountains form a biological island separated from any other mountains by desert flats. The surrounding flats vary from 2000 to 3000 feet in elevation. The highest point of the Chisos Mountains is Emory Peak with an elevation of 7835 feet (U. S. Geological Survey Topographic Map, 1905). Above 5000 feet the characteristic species are mainly those of the Arid Transition life zone. The common trees are pinyon, juniper (at least three species), oak (several species), some yellow pine, Douglas fir, aspen, Arizona cypress, little-leaf maple, and madrone. Photographs illustrating the character of the flats surrounding the Chisos Mountains and the higher parts of these mountains are shown by Bailey (1905), plates 5 and 11 respectively. On May 7, 1937, my attention was drawn to the wall of a narrow canyon by the squeaking of bats. Climbing to the top of a steep talus slope I came to the base of a cliff composed of volcanic rock, in which the bat colony was located. About 40 feet above the top of the talus was a horizontal crack in the rock which appeared to be about 6 inches wide and 20 feet long. There was no possibility of reaching the crevice, so I resorted to a load of fine shot which brought down three specimens of Tadarida macrotis. Collections taken from various parts of the crevice indicated the colony was composed entirely of females, most of which were pregnant, and that no other species was present. At the base of the cliff lay a deposit of guano which extended the full length of the fracture. It was three to four feet wide, and in some places six inches deep. This indicated that the crevice had been occupied for at least several seasons. The droppings, samples of which were saved, are coarse in texture, light in color, and irregular in contour and shape. The larger ones are five to six millimeters long by two to three millimeters in diameter. The evening of the day on which the bats were discovered I stationed myself in the canyon a mile below the colony and stayed there until after dark. During this vigil I saw no bats which appeared to be the large free-tails. In order to determine the approximate number of bats in the colony and the time of leaving the roost I returned to the site the next evening before sunset. A pair of white-throated swifts flew repeatedly in and out of one end of the crevice, where apparently they were nesting. At intervals the bats did a great deal of squeaking, especially when the swifts flew past the main entrance of the fracture. At 7:40 P.M. Pipistrellus began to forage and as it became darker other small bats appeared, but it was not until 8:20 that 65