.dropcap { color: #838078; float: left; font-size: 82px; line-height: 60px; padding: 5px 8px 0 0; } A burst of whistles pierced the foggy morning in the Missouri Ozarks. It came from Marina Rodriguez’s Bluetooth speaker but sounded enough like a Yellow Warbler’s song to catch the attention of two males. They swooped down from the high canopy and landed simultaneously in a mist net that Rodriguez and her team had erected alongside the Current River. Netting two birds at once is a rarity. “Kind of like Christmas,” said Rodriguez, a Ph.D. student at Colorado State University. “I think we got two because they were fighting over territory. Each thought the other was making the sound.” It was a promising start to the final stop in a two-week springtime tour. Rodriguez and three other students had been roaming the Midwest in a white Ram 1500 in search of breeding Yellow Warblers. In mosquito-thick Kansas, they had encountered tornadoes, hail, and...