The Scarlet Macaw’s last, best defense against wildlife poachers doesn’t look like much: just a ramshackle collection of tarps, makeshift tables, plastic five-gallon buckets, jungle hammocks, and a cook fire, hidden in the dense understory of a tropical hardwood forest near the fraught and uncomfortably porous border between Belize and Guatemala. It’s taken us hours to get here—the first leg an overland journey from San Ignacio in the Cayo District of western Belize, haggling our way through military and ranger checkpoints and bumping over red dirt roads that are more rock and ravine than actual thoroughfares. But reaching the banks of the Macal River was merely the first step. The trip upriver took us another hour or so, the labored whine of the skiff’s outboard motor following us as we passed hundreds of drowned trees jutting skyward from the water and verdant riverbanks. Egrets, cormorants, and Anhingas eyed us suspiciously while a Double-toothed Kite wheeled...