MEXICAN FREE-TAILED BAT. Nyctinomus mexicanus (Saussure)

Field characters.—Size medium, more than twice size of Little California Bat, decidedly smaller than Large Brown Bat. (See fig. 8 and pl. 21.) Total length about 4 inches (97-103 mm.), tail about 1-1/2 inches (32-40 mm.), hind foot less than 11/2 inch (7-12 mm.), ear about 1/2 inch (13-14 mm.), spread of wings 11-1/2 to 12 inches. Coloration dull dark brown above, paler on under surface; flight membrane, ears, and muzzle blackish. The terminal half of the tail projects behind the edge of the flight membrane (fig. 8), a character not shared with any other species of bat in the Yosemite region. Musty odor characteristic.

Occurrence.—Resident in Upper Sonoran Zone and lower part of Transition Zone on west slope of Sierra Nevada, where recorded at Coulterville, on Smith Creek (6 miles east of Coultervile) at El Portal, and in Yosemite Valley. One record for Walker Lake, east of Sierra Nevada. Forages high in open air apart from trees or other vegetation. 'Roosts' in buildings. Colonial.

The Free-tailed Bat is a bat of the open places, and in its route of forage and manner of flight more nearly resembles the swallows than does any one of the other local bats. It is to be looked for over fields, or else well above the level of the tree tops or brush. A distinctive feature of this species is the narrowness of its wings, a shape which is perhaps more useful to an animal flying swiftly in the open.

Fig. 8. Mexican Free-tailed Bat; from specimen, about 9/10 natural size. Dudley (six miles east of Coulterville), June 24, 1922.

This bat is highly colonial, sometimes being found in gatherings which include hundreds of individuals. It seeks by preference the attics of buildings rather than natural abodes in crevices in trees or cliffs.

In time of appearance the Free-tailed Bat resembles the Large Brown Bat. Thus, the two species appeared simultaneously at 7:30 P.M. in Yosemite Valley on July 24, 1920. At El Portal, January 1, 1915, a Free-tail was shot shortly after 5:14 P.M. This latter occurrence indicates not only that this species winters in the region, but also that individuals forage abroad in midwinter when weather conditions are favorable.

The single example from the east side of the mountains was picked up dead on September 12, 1915, at Walker Lake.

At Coulterville, on June 7, 1915, two members of our party visited several buildings in the town which were reported to be inhabited by bats. The specimens obtained in one of these places were all Free-tailed Bats, but whether all the buildings had been tenanted by this species could not be ascertained. The attic over the local drug store was a large affair open under the eaves so that bats could easily gain entrance. Only four individuals were seen there at the time of our visit; but on the floor of the place there was bat guano to the depth of 4 inches or more, indicating that a large colony had tenanted the place in previous years. Mr. W. J. McCarthy, who was a druggist there for many years, told us that he once burned some sulphur in this attic and killed "hundreds of bats." We found the village assembly hall to contain a few Free-tailed Bats crowded into a space about 2 x 4 x 36 inches beside a door casing, and others had evidently roosted along the ridge pole.

The Masonic Temple had suffered more extensively from the bats. In the low attic of this building were found only a very few individuals. But at some previous time there had been a large population, for guano lay in considerable heaps over the ceiling laths, and bat urine had stained the walls of the room beneath. We were told that "thousands" of bats had lived in this attic and that it had taken "an hour and a half for them to come out in the evening," but these multitudes were not present at the time of our visit. Guano left in the attic of such a building, sheltered from moisture, retains its characteristic odor for many years, therefore it does not give a wholly satisfactory clue as to the recency of bat occupation.

The two bats taken at Coulterville were both females and each contained one large embryo.