Bats against an orange sky

Photos by Jaymi Heimbuch unless otherwise noted

After a dusty drive over the dirt roads of a rice farm, the caravan of cars stopped. We all exited our vehicles and moved to the front of the line. Dragonflies flitted over the tall, waving stalks of rice and mosquitoes began to appear in clouds. Tour leader Corky Quirk dropped a large box filled with bottles of OFF next to the orange cones she had set out. This was the 50-yard mark. We stood and waited while the sun set, because 50 yards in front of us, we were about to see 250,000 Mexican free-tailed bats take flight.

bat face

Photo: USFWS Headquarters/Creative Commons

The Mexican free-tailed bat is one of the most widespread mammals in the western hemisphere. These tiny, 3.5-inch long flying mammals have a tail that is nearly half its body length, and while many species of bats have skin that creates a webbing between the tip of the tail and the body, called the uropatagium, the Mexican free-tailed bat's uropatagium goes only about half way up the tail. Hence the name, free-tailed.

bat tail

Photo: USFWS Headquarters/Creative Commons

They can be found in the southern half of the United States, throughout Mexico, Central America and into South America. In fact, it is called both the Mexican free-tailed bat and the Brazilian free-tailed bat because that is where they migrate to each year — the species summers in the northern part of its range and migrates south for the winter. However, exactly where it goes and how it spends its winters is not clear — though how it spends its summers certainly is. Here in the U.S., Mexican free-tailed bats are well known and loved — at least those who understand them. The states of Oklahoma and Texas have named the Mexican free-tailed bat the official state bat, and where this species roosts in large colonies, humans are drawn to watch the nightly spectacle of their emergence from roost to sky.

bats over the causeway

That is exactly why we were gathered next to a causeway in Davis, Calif. A happy accident of engineering made the underside of this bridge an ideal habitat for roosting Mexican free-tailed bats. The expansion joints offer three ideal features that bats look for in a roost: they retain heat, and this species loves heat; they are inaccessible to predators; and they offer tiny cracks perfect for these bats to wiggle into and feel safely hidden. It also helps that this causeway spans a large wetland, including the Yolo Bypass Wildlife Area. It is here that one of the largest wetlands restoration projects in the west has taken place and, with the recovery of native habitat comes an enormous food source for bats: insects. Mexican free-tailed bats eat as much as four times their own body weight in insects every single night. Their presence is a huge boon for local farmers, who benefit from the free pest control. In fact, a recent study showed that bats save farmers billions of dollars every year in reduced pesticide use and reduced crop damage thanks to their help with eating harmful insects. As problems like white-nose syndrome increase, so too does the cost of farming. Protecting bats directly translates to protecting our food supplies.

Corky Quirk, the tour leader, is a local expert on bats and runs Northern California Bats, a nonprofit focused on rescuing, rehabilitating and releasing bats. She also is a teacher, and volunteers with the Yolo Basin Foundation, providing tours to people curious about local wildlife. While the Yolo Basin Foundation offers a whole range of events and tours for wildlife, it is the bat tours that are perpetually sold out. When Quirk asked for a show of hands among the 30 or so attendees who had become a member of the foundation specifically to go on a bat tour, more than half the hands were raised (the other half were probably mostly guests of the members!) There is something both mysterious and enchanting about bats, especially when you know you'll get to see a quarter of a million of them take flight at once.

bat truck

After the sun had set but well before the light had gone, the first wave of bats began. For several minutes the bats streamed out of the causeway, exiting away from the bridge and over the fields, to the amusement and amazement of the onlookers. After awhile, the stream petered out, but that was only the end of the first wave — yet another wave of bats began a few minutes later. Cars and trucks continued to make their way across the causeway, lucky in their timing to also get to witness the event. One semi-truck gave a horn honk and a wave as it passed the thousands of bats and small group of staring people. The bats flew off into the darkening blue sky where they would hunt at an elevation of about a mile up for the rest of the night.

bats on a blue background

Yolo Basin Foundation isn't the only place where people gather to see the species emerge for the evening. Other areas where the colonies draw crowds are Bracken Cave near San Antonio, Texas, where nearly 20 million bats roost; and in a more urban setting, under the Congress Avenue Bridge in Austin, Texas. where 1.5 million bats emerge every night during their summer stay.  

many, many bats

Photo: USFWS Headquarters/Creative Commons

While abundant elsewhere, the population of Mexican free-tailed bats on the West Coast is actually experiencing a decline, and it is considered a species of special concern in California. Factors include problems such as pesticides, which reduce its prey source; predators including house cats and even fly strips; construction and renovation on homes or trimming trees where bats roost; and the fact that the species prefers to roost in large numbers in a few locations. This means that vast numbers can be killed in one swoop if someone decides to cut down a particular tree or, in the case of the Davis, Calif., colony, if there is construction on the causeway that they call home. Thankfully this particular colony has the Yolo Basin Foundation and many fans to look out for their well-being.

And if you've never seen the spectacle for yourself, you can check it out in the video below:

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Jaymi Heimbuch ( @jaymiheimbuch ) focuses on wildlife conservation and animal news from her home base in San Francisco.

Bats take flight in seasonal spectacle
Hundreds of thousands of Mexican free-tailed bats take flight from a causeway in California. Our intrepid photographer shares the spectacle.