The Franklin Boulevard Bridge is habitat to a maternal colony of approximately 40,000 Mexican free-tailed bats (Tadarida brasiliensis). Tadarida brasiliensis is also referred to as the Brazilian free-tailed bat. These bats migrate to the Franklin Boulevard Bridge in February and March, and use the bridge for day roosting. It is not known where they migrate from or where they go when they leave the bridge in October. It has been speculated that these bats migrate from the west coast of California (Wyatt).

The following description is taken from The Lives of Mexican Free-tailed Bats.

The largest populations of Mexican free-tailed bats live in Central Texas and Mexico, but they are also common throughout much of western North America, southward through Central America, and into the arid and semi arid regions of western and southern South America. They live in many habitats, including urban areas, and range from deserts to piñon-juniper woodlands and pine-oak forests. Although bachelor colonies of free-tails have been found at elevations over 9,000 feet, large nursery colonies tend to prefer relatively dry areas below 5,000 feet. Mexican free-tails typically live in caves, abandoned mines, or tunnels, and also roost in buildings, under bridges, in rock shelters, in hollow trees, and in cliff-face crevices.

Just before the northward migration, the free-tails will mate. Males apparently do not reach sexual maturity until their second year, while females as young as a year old have been found pregnant. Each female will typically give birth to just one young (called a pup). Virtually all the females give birth during a brief span of time, usually between the first and third weeks of June. Birth periods may vary from year to year, however, since weather differences can affect the length of gestation.

By summer, male and female free-tails will have divided into bachelor and nursery colonies. Bachelor groups are relatively small, consisting of dozens to hundreds of individuals, but can number 100,000 or more. In contrast, most nursery colonies are large, numbering from the hundreds of thousands to millions.

Mexican free-tails normally emerge by sundown. They typically feed from dusk until dawn. They spend more time traveling and feeding each night than most bats, in part due to competition from large numbers of roost mates. Nursing mothers require at least twice as much food as nonreproductive bats, especially as their pups near fledging. At such times, they may consume their body weight nightly. They feed exclusively on flying insects, mostly moths, flying ants, and beetles, according to samples thus far reported. A study conducted near Carlsbad Cavern, New Mexico, determined that about half of the insects eaten were pests that had fed on alfalfa and cotton crops, the nearest of which were grown some 40 miles away along the Pecos River.

Several species of myotis bats (Myotis sp.) are listed by the U.S. Fish and Wildlife Service as species of concern. These bats occur throughout California and prefer riparian habitats (within the Central Valley). Washes also provide important habitats in desert areas. Many species are known to occur in bridges, either colonially or alone.

Surveys conducted on the bridge structure by David Wyatt in 1999 suggested the possibility of three different species of myotis bats utilizing the bridge. These are the California myotis (Myotis californicus), the small-footed Myotis (Myotis ciliolabrum), and the yuma Myotis (Myotis yumanensis). In order to lessen the amount of disturbance to the colony, bats were not handled during these surveys, and, therefore, the presence of myotis bats in this colony could not be confirmed.

During the month of April 2000 there was one small bat noted in Bat House #2 that may have been a myotis bat.

Big brown bats (Eptesicus fuscus) have been observed on the Franklin Boulevard Bridge in both 1999 and 2000. These bats are larger than the Mexican free-tails and are easily identified from a distance by their size, dark brown fur and black markings. The bat's naked parts of the face, ears, wings, and tail membrane are all black.

The following information is taken from the University of Michigan web page:

Eptesicus fuscus is an insectivorous bat. It preys primarily on beetle using its robust skull and powerful jaws to chew through beetle's hard chitinous exoskeleton. It also eats other flying insects including moths, flies, wasps, flying ants, lacewing flies, and dragonflies.

The big brown bat must confine its feeding activity to warm months when prey insects are active. Therefore it has to accumulate enough fat reserves, as much as one third of its body weight, before entering hibernation. Some estimate that these bats catch at least 1.4 grams of insects per hour. Another study identified a single adult that gorged on food at a rate of 2.7 grams per hour.

Like most other bats, Eptesicus fuscus does not feed in heavy rain or when the air temperature dips below 10 degrees centigrade. In good weather they will begin foraging 20 minutes after sunset. They eat until full, and then often make use of a "night roost". This means the bat will hang under a porch or in a barn to rest while digesting its meal. It returns to its day roost before dawn.

Sperm is stored in the female tract until the beginning of April, at that time ovulation and fertilization take place.

The female gives birth to one or two young after a 60-day gestation period. At birth, the young is blind, naked, has closed eyes, and weighs about 3.3 grams. The young grow rapidly, their eyes opening after about seven days. The young are weaned and ready to fly by late June or early July. They may reach adult size in August. Males are not involved in parenting.

Female big brown bats form maternity colonies to rear young. The size of these colonies range from 5 to 700 animals. The males roost alone or in small groups during this time. Both sexes will roost together again in the late summer.

In late pregnancy, the females are reluctant to fly. Newborn young are not carried by the mothers during feeding flights, instead they are left behind in a cluster. Apparently, the mother only moves the young to transport them from one day roost to another.

This bat can survive up to 19 years in the wild and males tend to live longer than females.

Bats spend more than half their lives in their roost environments (Nowak 1994). The Franklin Boulevard Bridge is used for both day roosts and night roosts. It is used by the Mexican free-tails as a day roost to rest and rear their pups. These bats use the small crevices in the expansion joints. Hundreds of bats have been observed in areas as small as1" wide, 12" deep and 2' long. The larger open areas between the bridge support beams are used for night roosts. This is evidenced by both Tadarida brasiliensis and Eptesicus fuscus guano found during the day when bats are not present. In some areas that are larger than the typical small crevices used by the Mexican free-tails, but smaller than the large open areas, both the big brown bats and Mexican free-tails were observed roosting together during the day.

The Mexican free-tails can enter areas that are no more than ½" wide. In some areas that had been caulked to exclude them we could hear them during the dusk period prior to their evening flight. The caulking had pulled away slightly from the edges of the expansion joints and they had entered. While we could not view them directly with spotlights due to the height of the area and the depth of the crevices, we observed them flying out at dusk. Their ability to occupy a space seems limited only by the size of their skull. In some of the rail beams, on the side of the bridge, there are tiny openings at the bottom that we did not bother to caulk. However, a light beamed up into this area at night revealed a little bat face peering down at us producing a squeak at the disturbance.

Most of the bats on the Franklin Boulevard Bridge have been observed near the Bean Ranch Road area. This is where the crevices are narrow due to a curve in the roadway making the area an attractive habitat to the free-tails.

Removal of the bridge will eliminate the primary roosting habitat for the colony of Mexican free-tailed bats, big brown bats and possibly myotis bats. Although the new bridge will include expanded habitat for this colony, it will not be available for up to two seasons.

Bat protection is supported by the agricultural growers in the project area and the Agricultural Extension Office. Since the bats consume their body weight in insects nightly (at approximately 12.3 grams each, 60,000 bats will eat almost a ton of insects every night) they significantly reduce the amounts of pesticides required in the fields.

According to Bat Conservation International, one bat can easily eat 20 female corn earworm moths in a night. Each moth can lay as many as 500 eggs, potentially producing 10,000 crop-damaging caterpillars. Yet as few as 8 caterpillars per 100 plants can force a farmer to apply pesticides, demonstrating the impact of even small bat colonies.

Bats do not appear to affect the structural integrity of bridges. During nationwide surveys, no structural damage was attributed to bats during the normal life span of concrete structures. In addition, there is no evidence that large bat colonies degrade the environment. Two water quality studies conducted on Town Lake beneath the Congress Avenue bridge in Austin, Texas found a negligible impact caused by the guano deposits. The Congress Avenue bridge has 1.5 million bats.