When zoology professor Daniel Baldassarre traveled to the remote Galápagos Islands of Wolf and Darwin with two other researchers, getting there proved quite a challenge. With no beaches to land a boat on, they had to approach the islands’ cliffs in a small dinghy and wait for a lull in the waves before jumping onto sharp, black lava rocks.

But their efforts paid off. On that trip, they saw a bird that very few people have ever set eyes on in the wild: the vampire ground finch. These finches are only found on Wolf and Darwin , and feed on the blood of much larger birds called Nazca boobies that also live on the islands.

Up close, Baldassarre and his colleagues noted the vampire finches’ blood-stained bills and their throats visibly engorged with blood.…