For half the year, a little brown bird on the northernmost islands of the Galápagos uses its wickedly sharp beak to pick at seeds, nectar and insects. But when the climate dries out, it drinks blood.

Yes, there is such a thing as a vampire finch.

Yes, it is what it sounds like.

Galápagos finches have been used since Darwin’s time to illustrate evolution in action. Even among them, Geospiza septentrionalis is an outlier, one of the few birds in the world to intentionally draw and drink blood. And the species is only found on Wolf and Darwin islands, two of the most remote and off-limits places in the entire archipelago.

The vampire finch has a method. First, one bird hops on the back of a resting Nazca booby, pecks at the base of the seabird’s wing, and drinks. Blood stains the booby’s white feathers.

Other finches crowd around to wait their turn, or to watch and learn. Because adult boobies can fly away, the attacks are almost never fatal. The only casualties are chicks that flee from the finches on foot and, unable to find their way back, starve.