NightWings Over Yolo Bypass

Mexican Free-Tailed Bats at the Yolo Basin Wetland

 If you happen to be in Northern California on a hot summer night driving between Davis and West Sacramento you might be lucky enough to see one of nature’s grand events. As the sun is setting and the tree swallows, crows and black birds are flying home to their roosts after the long day in the marshlands, a large colony of 250,000 Mexican Long Tailed bats is beginning to stir 30 feet off the ground in the long narrow slits of the cement eight lane Interstate 80 that runs east to west over the wetlands.

   The Mexican Free-Tailed weigh about 15 grams is 3 ½” to 4 3/8” inches long, with a 12 1/2" wingspan. It has light to dark grey fur, black wings, and small dark eyes. With very tiny but sharp teeth and the usual bat radar, which enables them to home in on their prey they can eat one third of their weight in one good nights hunting. I learned all this information from Corky Quirk, the top bat researcher at the Yolo Wetlands. Corky works with the Department of Fish and Game out of a field office on the east side of Davis and just down the street from the official entrance to the preserve. Corky also runs the group Northern California Bats. As their websites states, “ NorCalBats is dedicated to the rescue, rehabilitation and release of bats throughout Northern California. In addition, we are committed to public education regarding the environmental benefits of bats and dispelling fears and myths that lead to the death of roosts and colonies.”

­ During the late summer months, inside the air-conditioned station, Corky gives a half hour lecture on the Mexican Free-Tailed bat. Known in Latin as Tadarida brasiliensis, they are of the Order Chiroptera. Chiroptera, "hand wing" which alludes to the great elongation of the fingers that support the flying membrane. The Mexican Freed-Tailed gets its name from a naked tail extending well past the edge of tail membrane and has a little wrinkled upper lip with a snubbed nose.  Spoon-shaped bristles on the hind toe are used to groom themselves, and thumb and toe claws have double talons. Their wings are narrow, and flight is straight, rapid and aggressive. The officials running the wetlands want the visitors that wish to see the grand exit of the bats for the evening to car pool and form a tight line out into the preserve. Sometimes they have up to fifty cars heading out at sundown to see the bats fly out so the officials like to keep the traffic orderly and as small as possible in order to cut down on the impact inside the wetlands.  The bats come out in three giant waves, one at a time.  They drop down as a group and then head out in a river of bodies over a eucalypts tree that stands along-side the causeway. These clusters number approximately 250,000 and fly out at 35 miles per hour, and then increase in velocity to 40 to 60 miles per hour. Traveling in a line then up at a steep angle that stretches out for at least a quarter mile and turns into a swirling mass at about five thousand feet, where they then fan out over the vast dinner table that is the wetlands and farmlands below. Corky has collected up to 50 bats that have fallen from their perches. The bats need to drop down from a high spot in order to fly and if they should hit the ground they will usually not be able to take off again. Unless someone like Corky comes along to rescue it, a bat will become part of the food chain for the local snakes, raccoons, insects and other predators. Corky also says that, “one out of ten bats found on the ground can carry rabies, so it is always best to handle them with thick gloves and to have a box or bag to place them after you capture them as they ‘will’ bite you even if they are not ill with rabies and they have very sharp teeth that will penetrate your skin so be careful!”

   In flight, they might remind one of swifts, with the long, angular, and narrow wings, and their relatively large size, make them easy to identify. Bats form huge colonies of various sizes like this colony of approximately 250,000 in Northern California to an estimated 30,000,000 in the Bracken Cave in Texas. The habitat of the Mexican free-tailed bats is varied.  In the West and Southeast they live mainly in buildings and in the south they are dwelling mostly in caves. Several million bats live in Carlsbad Caverns, and larger nursery colonies live in Mexico, Arizona and Texas.  The species is one of the most widely distributed mammalian species in the Western Hemisphere. Northern limits extend from southern Oregon, Nevada, and Utah to Nebraska, Mississippi, to southern North Carolina. From this northern swath, the Mexican Free-tailed Bat ranges southward through most of Central America, Antilles, through most of western South America to central Chile and east to the coastal provinces of Brazil.

   Their roost under the causeway is the narrow two inch slits that run the length of the underside of the structure where the little bats climb into and hang upside down safely residing without any major predators.  The close access to the wetlands and agriculture areas with millions of insects is also a key factor in their choice for ‘hanging out in this area’. The colonies are a major form of agricultural pest control, consuming literally tons of insects each night. It's been estimated that the bats of Bracken Cave in Texas alone can eat 250 tons of insects in a single night. The bats fly high in the sky to feed on flocks of migrating moths, saving crops from millions of corn rootworms each year.  Another reason these bats hang out under the causeway is to rear their babies.  Each female bears a single pup in June to mid-July. Coming out just as soon as the sunsets and the bugs come out and return at first light at which time they return to their perches under the causeway.  In the east and on the west coast this bat hibernates but does not migrate; bats in Texas and the southwest usually migrate to Mexico in October and return in March.  Older males may remain on the wintering grounds year-round. Young hang together in a nursery, and mothers return and feed any of them.  The lifespan is usually 13-18 years for males and 12 for females, who produce 1 offspring per year.