It doesn’t look promising. They come every evening, we’ve been told. As sure as sunset. You can practically set your clock by them . . . though there are always exceptions. A wise caveat, as sunset is upon us and the only creature in sight is a black-billed mountain toucan noisily shelling seeds in a broadleaf evergreen tree. What’s more, as the thin mountain light fades, practical matters loom. Having slithered a mile down a steep Andean clearing to reach this spot, we’ll soon be scrambling back up in the dark. We are six—the driver, three biologists from the Colombian nonprofit ProAves, the Ecuadoran photojournalist Pablo Corral Vega, and me. Between us we can muster a headlamp and two cell phones with which to light our way back to the jeep. It’s pointless to wait. They’re not coming. We should go. And yet we linger. The evening’s magic has transformed this uneven, boggy clearing. The stillness. The green-scented mountain air. The ghostly ivory columns of the...