MOORHEAD — Scientists in northern Minnesota are trying to decode frog sounds in hopes of understanding why the amphibians’ populations have been shrinking.

Frogs have been dying off because of disease, pesticides and habitat loss. But researchers didn’t know exactly how significant each factor was, nor could they study frogs by watching the elusive amphibians. So, they listened to frogs instead.

Scientists have set up recording devices in 34 sites across North America to collect five minutes of sound every hour, seven days a week. One site is in the Tamarac National Wildlife Refuge in Moorhead, where scientists have been gathering data for seven years across 10 wetlands.

By analyzing the data, researchers have been able to quantify the population die-offs. Between 2002 and 2011, populations of at-risk frog species fell nearly 12 percent. However, scientists were surprised to find that populations of all frogs, including those considered healthy, were down nearly 4 percent, reports say.

Michael Adams, a research ecologist with the U.S. Geological Survey, said the results add to scientists’ concern for what’s happening to the amphibians.