WAYNESBORO, Virginia—Standing in the woods along the South River, Kelly Hallinger held the microphone up to capture the cacophony of songs, one at a time: the urgent, effervescent voice of the house wren; the teakettle whistle of the Carolina wren; and the sharp, shrill notes of the song sparrow.

It was the summer after her freshman year at the College of William and Mary in Williamsburg, and Hallinger was working with her professor, ecologist Dan Cristol, to investigate the effects of mercury left behind by a factory. Over and over she recorded birdsong, visiting various sites in the woods and along the shore, some polluted, some unpolluted.

When she got back to Williamsburg with her tape recorder, Hallinger