Andrea Morales Rozo, who teaches biology at the Universidad de los Llanos in central Colombia, guided the team at the nets, from which she skillfully extricated birds unharmed. Ms. Morales Rozo has been studying the blackpoll warbler, a species that migrates between the Amazon and Canada; she was part of a group that recently compared museum specimens and field-caught birds and learned that the warbler’s northward range had shifted by nearly 400 miles in 45 years.
Dr. Cuervo, the expedition leader, offered calm, fatherly support to those at the processing table. It’s not always obvious how best to describe a bird’s colors, for example, and second opinions were often requested. Was a wing “verde café,” greenish brown? Or was it “verde olivazo,” olive green? Was a female bird’s brood patch, the bare skin that warms the eggs, still smooth or becoming wrinkly?
MOR-001 struggled in Ms. Soto’s hand as she passed it to her colleague, Jessica Díaz, a field biologist hired for the expeditions. The bird was photographed and logged. Ms. Díaz labored to extract a tiny amount of blood from its jugular vein with a syringe, expressing the drops into a vial of alcohol. She then prepared herself to euthanize it with rapid cardiac compression, using fingers to apply firm pressure over the bird’s heart. With this technique, small birds pass out within seconds and die in about half a minute. Large birds are anesthetized.
Ms. Díaz held MOR-001 under the table so as not to have to watch; her colleagues did the same whenever their turn came to sacrifice a bird. “This is the not-fun part,” she said, softly.
A few in the group, including Ms. Soto, avoid sacrificing birds, although they believe in the necessity of scientific collecting and participate in the process. “I think it’s hard on all of us,” said Ms. Soto, whose high, mellifluous voice gave her a certain birdlike aura. “But it’s really hard on me. It just stabs me through the heart.” On this expedition, Ms. Soto assumed other jobs on the assembly line: cutting samples of pectoral muscle to drop into liquid nitrogen, calling out colors of beaks and feathers, gingerly tagging a leg.