Boreal chorus frog (Pseudacris maculata) is one of the smallest frogs in Minnesota. It is found throughout the state in shallow wetlands, shallow parts of lakes, temporary pools, and grasslands near wetlands—always near woodlands. With their short legs they are not very acrobatic and do not climb well. They are the first frogs to appear in spring, emerging in late March or early April when snow and ice may still be present. Males call from when they first emerge to late July. Their distinctive call sounds like a person running their thumb across the fine teeth of a pocket comb.
http://www.minnesotaseasons.com/Amphibians/boreal_chorus_frog.html