Bats' Wintering Sites

Preliminary Survey of Wintering Sites of Brazilian (Mexican) Free-Tail Bats from Carlsbad Cavern

Some very basic facts regarding the long-distance seasonal migration of Brazilian (or Mexican) free-tail bats are generally understood. Most of these bats arrive at summer roosts, such as Carlsbad Cavern, in March and April, and depart for southern locations in Mexico by late October.

But these basic facts leave quite a lot of room for questions. Where exactly do the bats go in Mexico? Do they more or less group together? How far do they migrate in any given night? Where do they stop over and how far south do they eventually go?

The fact is that despite being one of the most numerous mammals in the Southwest, the whereabouts and status of winter populations of these animals is still largely unknown. Learning the answers to these questions is of great interest for ecologists and natural resource stewards, who cannot develop sound conservation strategies without some knowledge of the bats’ winter habits. And even the best protective measures put in place for bats in the park during the summer months cannot protect them once they leave for their winter ranges.

We would probably know even less about our wintering free-tail bats were it not for the fact that some bats are hosts to the rabies virus. The discovery of rabies in bats in 1953 led to the funding of numerous ecological investigations of insectivorous bats by the US Public Health Service. This research included banding of large numbers of free-tail bats at Carlsbad Cavern and other locations in New Mexico, Arizona, Texas, Oklahoma, and Mexico. Conducted during the late 1950s and 1960s, the study established multiple connections between bats at Carlsbad Cavern and other localities, including several winter records from Mexico.

In recent years, biologists made three separate trips to numerous states in Mexico, including four known wintering sites for Brazilian free-tail bats from Carlsbad Cavern to verify that the caves are still used by the bats. In December 1999, they searched four caves in the states of Nuevo León, Jalisco, Durango, and Sinaloa. In December 2000, seven caves were investigated in Nuevo León, Tamaulipas, San Luis Potosí, Michoacán, Colima, and Jalisco states. In February 2001, they searched in the state of Sonora but couldn’t find the cave.

Out of all this searching, only three winter roosts of Mexican free-tail bats were found. One was a large winter roost of approximately 100,000 bats at Cueva de La Boca, in Nuevo León. Another roost of about 10,000 free-tail bats was found in Grutas de Quintero, in Tamaulipas. Cueva de La Isla Janitzio, in Michoacán, also had a small colony of free-tail bats.

Several other caves, especially those farther to the north, showed evidence of being summer roosts for free-tails. This might mean that those bats also fly farther south for the winter. These efforts and associated findings illustrate how little is still known about the winter habits and migration of free-tail bats.