MARANA -- In warmer months, 30,000 bats sleep the day away under the bridge carrying Ina Road across the Santa Cruz River northwest of Tucson.
As the sun sets, the bats emerge in droves from tiny gaps between the concrete slabs supporting the span, creating an aerial spectacle that draws crowds.
The community's enthusiasm for the bats is one reason Janine Spencer, environmental projects coordinator with Marana, was concerned when the town and the Arizona Department of Transportation announced plans to replace the bridge and by doing so rid the migratory bats of their summer roost.
"Unfortunately, the new bridges don't have the crevices that bats need," Spencer said. "And we have so many old bridges in Tucson that are just fabulous bat habitat. But the new ones are flat-bottomed."
The Ina Road bridge has been deemed structurally unsound and will be replaced with two bridges, eastbound and westbound.
In an effort to reconcile the need for a new bridge with the potential loss of habitat, Marana and the Arizona Game and Fish Department will equip one of the spans with bat roosts. Spencer and other bat experts successfully requested funding for the project from Pima County's Regional Transportation Authority.
The structures, called bat condos, will be 14 inches deep and 48 inches long. They will contain crevices, similar to those found in the current bridge, that will vary in width from a half inch to 1.5 inches to give bats a range of options. The condos will boast attic space and vents to allow for airflow.
The nine roosts, to be installed under the eastbound bridge, will accommodate about 30,000 bats. Spencer has worked with engineers in Arizona and California to design the condos.
"The usual thing that people can do in a lot of parts of the country is just hang up some bat boxes," Spencer said. "We can't really do that here because it's too hot in the summer and it's too cold in the winter. So we had to find something."
Of the $80,600 requested, the RTA so far has approved $30,500 for the first phase of the project, which will begin this summer. The money will come from the RTA's $45 million Wildlife Linkages Fund, approved by Pima County voters in 2006, that is fed by a 0.5-cent transaction privilege tax.
Jim DeGrood, director of transportation services for the Pima Association of Governments, which is an agent for the RTA, said the Wildlife Linkages Fund strikes a balance between infrastructure upgrades and environmental preservation.
"This was an opportunity for us to work with the environmental community, addressing things that were a real impediment to wildlife movement when we were developing road projects," he said.
Beginning this summer, the Arizona Game and Fish Department will monitor the Ina Road bats and their habitat for a year before construction starts to ensure the bat condos mimic the animals' current environment.
The first phase will also include installation this fall of two smaller condos, each of which will house 2,000 bats, under the Cortaro Road bridge a mile north of Ina Road. That bridge is a new, flat-bottomed bridge, so few bats roost there. The hope is that when the Ina Road bats are temporarily displaced by construction, the Cortaro Road condos will give some of them a place to roost.
The second phase, starting with construction of the eastbound Ina Road bridge and the bat condos, is slated to begin in fall of 2015.
The three bat species that roost under the Ina Road bridge are migratory, and many of the bats are pregnant females. Although they aren't endangered, the bats balance the southern Arizona ecosystem by eating crop pests and pollinating the native saguaro and agave plants.
Providing bats a safe place to roost is crucial to maintaining healthy population numbers, said Ted Fleming, an emeritus professor of biology at the University of Miami who has also taught at the University of Arizona.
"We don't really know what controls bat population numbers," he said. "Suitable and safe roost sites have to be a major ingredient in having healthy bat populations."
Fleming called bridges a "fallback" roost for bats because they prefer caves and mines. But a dearth of these habitats in Arizona makes the Ina Road bat condos crucial for conservation, he said.
"Bat condos are a very effective way of providing alternate roosting places for the bridge-living bats here in the Southwest as well as the Southeast," he said.