For half the year, a small brown bird on the northernmost islands of the Galapagos uses its wickedly sharp beak to gather seeds, nectar and insects. But when the weather dries up, it drinks blood.
Yeah, that's right, a vampire finch.
Little birds of the Galapagos Islands 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 drink blood. And the species is only found on Lobo and Darwin Islands, two of the most remote and forbidden places in the entire archipelago.
The vampire bird has a method. First, a bird hops on the back of a resting Nazca atobah, pecks at the base of the seabird's wing, and then feeds on the blood that stains the white feathers.
Other finches huddle around to wait their turn, or to watch and learn. Because adult atobahs can fly away, attacks are almost never fatal. The only fatal victims are nestlings that run away from the finches on foot and, unable to find their way back, starve to death.
Drinking blood is an unusual diet, and research published last year showed that vampire finches have evolved specialized bacteria to aid in digestion. Even more surprising, according to a paper published this week in the Royal Society B journal Philosophical Transactions, is that some of these bacteria are similar to those found in the haematophagous bats of theCentral and South America.
Jin Song, a biologist at the University of California at San Diego who is the lead author of the study, has previously studied the convergent evolution of gut bacteria. Do different animals with the equivalent of diets of the common - eating only ants and termites, for example - develop a similar gut microbiota over evolutionary time?
Vampire finches, which were first discovered in 1964, gave Dr Song the chance to look through the guts of blood drinkers from different branches of the tree of life. "When I found out about vampire finches I was very shocked," he said. 
Chaffinches line up to drink the blood of a booby. Image: Jaime Chaves
Blood-drinking finches are extremely rare. They only resort to their vampiric diet in hard times, as blood has dangerously high amounts of salt and iron - and is poor in essential nutrients such as B vitamins. Some types of bats face the same dietary challenges.
Dr Song had already collected data on haematophagous bats. But to compare these animals with birds, she had to turn to colleagues working in the Galapagos Islands, who collected samples of the animals' faeces.
When Dr. Song's team compared the bacterial genomes of vampire poop to the bacteria in the guts of hematophagous bats, they found few similarities. But, as the team showed in their paper, the two gut microbiomes had one ingredient in common that could help digest blood: high levels of Peptostreptococcaceae, a group of bacteria that helps processsodium and iron.
Given that these bats and birds followed very different evolutionary paths on the way to their blood-drinking lifestyle, "It was interesting that we could find something that they had," Dr. Song said.
"The analyses are very good," Rosemary and Peter Grant, biologists at Princeton University, said by email. The two have studied Galapagos finches since the 1970s.
They also saw another strange extension of the same feeding habits, they said. "Ground finches were observed drinking blood from the placenta of a sea lion that had just given birth a few times."
Back in the Galapagos Islands, Dr. Song's co-authors, Jaime Chaves of the University of San Francisco in Quito, Ecuador, and Daniel Baldassare, a fellow biologist, are testing whether the finches have also developed any painkilling substances or anti-clotting proteins that vampire bats use on their victims.
Dr. Chaves still marvels at the "privilege" of seeing vampire finches in the act.
"It's one of the most rewarding things for any scientist to be able to witness this unique behavior," he said.
