SWARMED BY MOSQUITOES AND carrying 10-foot metal poles, biologists Erik Johnson and Olivia Butler splash through a swampy nature preserve in Lafayette, Louisiana, on a mission. A concert of bird calls rings out, but the Audubon Delta scientists listen for one in particular: a lear, high-pitched sweet, sweet, sweet, sweet—the tune of the bright yellow Prothonotary Warbler, or “swamp canary,” as they’re nicknamed in Louisiana.
Stopping near a nest box, Johnson stretches a net across the poles while Butler places a yellow rubber duck—the perfect color and size to act as a decoy—nearby. She then plays a variety of warbler calls on a speaker, trying to lure a bird. For 15 minutes, flashes of yellow swoop from branch to branch—until finally, one flies into the net. “Yes!” Butler whispers. The…