On a late spring morning, Christy Morrissey, a wildlife ecotoxicologist at the University of Saskatchewan, drove her mud-splattered pickup past an undulating sea of cultivated fields two hours north of Saskatoon. Now and then, the land rose to form low green hills, and the sound of aspen leaves rattling in tall-grass sloughs floated in the open windows. All the way to the horizon, shallow ponds, called potholes, punctured the landscape, each dotted with ducks and ringed with bulrushes and cattails. After inspecting a map drawn by a student, Morrissey pulled to the side of the gravel road, grabbed a small cotton bag, and crept stealthily toward a wooden box nailed to a fence post. Like a pouncing cat, she clapped her bag over its round entrance hole, and then cautiously opened the box’s side door. Staring into the middle distance, she felt around inside. “Got her,” she said, extracting a glossy Tree Swallow and gently tipping her head-first into the bag. “Fifty percent of...