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.
Drinking blood is an unusual diet, and research published in 2018 showed that vampire finches have evolved specialized bacteria in their guts to aid digestion. Even more surprising, according to a paper this week in the journal Philosophical Transactions of the Royal Society B, is that some of these bacteria are similar to ones found in the vampire bats of Central and South America.
It is one of the most rewarding things for any scientist to be able to witness such unique behaviour
Se Jin Song, a biologist at the University of California San Diego and the study’s lead author, had previously studied the convergent evolution of gut bacteria. Do disparate animals with the equivalent of fad diets — eating only ants and termites, for instance — develop similar gut microbiota over evolutionary time?
Vampire finches, which were first spotted in 1964, provided Song with a chance to look across the guts of blood-drinkers from different branches of the tree of life. “When I found out about vampire finches I was pretty shocked,” she said.
Blood-drinking finches don’t have it easy. They only resort to their vampiric diet in lean times, and blood is dangerously high in salt and iron — and low in essential nutrients such as B vitamins. Vampire bats face the same dietary challenges.
Song already had collected data on vampire bats. But to compare these animals to the birds, she had to turn to colleagues working in the Galápagos, who collected samples of vampire-finch feces.
When Song’s team compared the bacterial genomes in vampire finch feces to the bacteria in the guts of vampire bats, they found few close similarities. But as the team showed in their paper, the two gut microbiomes did have one ingredient in common that could help with digesting blood: high levels of Peptostreptococcaceae, a group of bacteria thought to help process sodium and iron.
Given these bats and birds followed very different evolutionary paths on the way to their blood-drinking lifestyle, “it was still interesting that we were able to find something that they did share,” Song said.
“The analyses are done very well,” said Rosemary and Peter Grant, biologists at Princeton University, in an email. The two have been studying Galápagos finches since the 1970s.
They’ve also seen another strange extension of the same feeding habits, they said. “Ground finches are sometimes observed drinking blood from the placenta of a sea-lion that has just given birth.”
Back on the Galápagos, Song’s co-authors, Jaime Chaves of the University of San Francisco de Quito, in Ecuador, and Daniel Baldassare, a fellow biologist, are testing whether the finches have also evolved any of the pain-numbing or anti-clotting proteins that vampire bats use on their victims.
Chaves still marvels at the “privilege” of seeing vampire finches in the act.
“It is one of the most rewarding things for any scientist to be able to witness such unique behaviour,” he said.