Yield: Frogs crossing.

On warm, rainy nights over the next few weeks, Cornell University biology students and members of the campus Herpetology Society will gather along a stretch of road in the Ringwood Preserve, about six miles from campus. Their mission: to stop and slow down automobile traffic, giving frogs and salamanders right of way to cross from the forest to a mating pond.

Millions upon millions of adult amphibians are killed on the first warm, rainy nights of spring as they cross roads throughout the Northeastern United States in their migration from breeding grounds to breeding ponds.

The Cornell students want to make a small contribution to end this slaughter between now and May 31, the end of the breeding season. "As a class and as conservation-minded herpetologists, we plan to decrease the number of animals killed this spring by assisting them to cross the road from the forest to where they reproduce," says Jacqualine B. Grant, a graduate student in neurobiology and animal behavior. "We want do this as safely as possible for them and for us."

A few months ago, students from the society and from the university's herpetology class, taught by Harry W. Greene, Cornell professor in ecology and evolutionary biology, contacted Ward A. Hungerford, superintendant of the local Tompkins County Highway Department, to explain their idea for a frog-crossing project. The department has cooperated by lending the group full-coverage reflective vests, slow/stop paddles, reflective barrels and traffic cones, work-zone signs, flagger signs and "be prepared to stop" signs. And county road technician Jerry Stern has marked the road so the students will know where to place their signs. The highway department has even trained the frog-crossing team in safe night-flagging techniques. The students have handed out leaflets that explain to nearby residents why traffic on Ringwood Road is slowing down. With everything in place, the students need one more ingredient to make this project a success: "Now, we are just waiting for it to rain," says Grant.

The Ringwood wetlands preserve is surrounded by neighborhoods, and the county estimates that Ringwood Road carries about 600 vehicles a day. The preserve features a wide variety of amphibians, including frogs called spring peepers – for their constant nocturnal cacophony – wood frogs, spotted salamanders, Jefferson salamanders and newts.

"Long before humans built a road, there was a pond. And all these pond-breeding amphibians need water to lay their eggs," says Kelly R. Zamudio, Cornell assistant professor of ecology and evolutionary biology. "If they don't lay their eggs in the pond, the eggs dry up. The eggs develop into tadpoles, the tadpoles develop into froglets, and then, as frogs, they leave the pond. This migration is not something they have choice about."

The frogs' and salamanders' reproduction is critical, says Zamudio. Most creatures do not migrate across roads to breeding areas en masse like this. If something goes wrong, or if too many get killed, large numbers of other individuals could be at reproductive risk. Spring peepers live in forests and feast on mosquitoes and flies. Each spring, the peepers climb down from the trees and travel to water, where they meet other peepers, court, breed and lay eggs, says Grant. "This whole procedure takes place in a relatively short period of time," she says. "What this means is that hundreds or even thousands of frogs or salamanders may emerge on a single rainy, warm, spring night and cover the roads on their way to the pond."

This mass emergence is a double-edged sword. "On the one hand, it allows us to go out and save a large number of amphibians all at once, but on the other it means that a large number will be crushed by cars in that same time period," says Grant. "It's in our best interest to conserve these amphibians because they eat insects, such as the disease-carrying mosquito."