"A healthy white-footed mouse population is going to mean we're going to have healthy bird population," she said. "We're going to have a healthy fox population."
Callaghan takes extra steps to make sure the trapping process is safe.
"We give them a little bit of bedding that they can chew up and make a nest with to stay warm in the evenings."
When it's time to check the traps she describes it like Christmas - you're opening a present and you're never sure what's going to be in it.
She recently captured a northern short-tail shrew, something she had been hoping to find for a while, "We've found them all around the parks in the past but we've never actually documented them within the park."
Now she has hope that wildlife that need the shrew as a food source can live a healthy life right in the city.
You can learn more about Milwaukee's wildlife at HKE MKE. It's a benefit hike for the Urban Ecology Center. The 2.5 mile hike is scenic and family-friendly. It is Sunday, September 17th. Get more information here.