Why Some of Darwin’s Finches Evolved to Drink Blood

Scientists suggest the vampire finch evolved to drink blood to survive the volcanic archipelago’s harsh environment and scarce resources

Vampire finch sips nectar from yellow flower blossom, appearing camouflaged into the branches
Vampire finches will resort to drinking blood for survival when they can't find other food sources like seeds and insects. Peter Wilton via Flickr under CC BY 2.0

The Galápagos Islands are home to 13 different Darwin's finch species that evolved from one common ancestor. Each of these finches adapted to their environment and adjusted their diet accordingly. Some finches prefer a modest diet of seeds, flower nectar, pollen and insects. Other finches prefer to drink the blood of large sea birds.

On Darwin and Wolf islands, part of a large marine sanctuary on the Galápagos archipelago, there lives an unlikely oddity: a blood-sucking finch. First spotted in 1964, the vampire ground finch, Geospiza septentrionalis, uses its razor-sharp beak to pierce the wings of a large sea bird called the Nazca booby, Sula granti, and drinks its blood. The odd behavior has fans of the BBC’s new David Attenborrough nature documentary, “Perfect Planet,” fascinated with the so-called vampire finches, reports Ibrahim Sawal for New Scientist, with many left asking: but why do they do this?

Drinking blood may seem like an unusual diet for the finches, but considering the finch's ability to adapt, it is not too surprising. Finches likely arrived on Darwin and Wolf islands 500,000 years ago, and have managed to make it work ever since. Darwin's finches have been studied since Charles Darwin, an English naturalist best known for his contributions to evolutionary science, first arrived on the Galápagos Islands in September 1835. Darwin observed the differences in the finches' diet on various islands and later observed their beak sizes. Beak size changed as the finches developed different tastes for available food. Island isolation often forces the finches to adapt to available food resources.

Because the islands are remote, even compared to other islands in the archipelago region, they are extremely harsh to live on, where food can disappear during the dry season. Vampire finches living alongside sea birds, including the red-footed and Nazca boobies, resorted to eating parasites that resided on these large bird's feathers and skin. The finches likely got a taste for blood when removing the parasites created open wounds. Eventually, the finches learned how to access blood by picking away at larger birds' wings and drinking it.

Vampire finches will resort to drinking blood for survival when they can't find other food sources like seeds and insects, writes researchers Kiyoko Gotanda, Daniel Baldassarre, and Jaime Chavez to the Conversation. But blood is low in necessary nutrients and too high in salt and iron, Joshua Sokol reported for the New York Times in 2019, so it’s more of a stopgap solution for food scarcity.

The Conversation authors were part of a team that found the vampire finch can survive on blood when resources are low because of unique bacteria other carnivorous birds and reptiles have in their guts. (Their work was published in the journal Microbiome in 2018.)

Building on their work, a 2019 study led by Se Jin Song, a biologist at the University of California San Diego, found that vampire finches and the vampire bats have a type of gut bacteria in common, Peptostreptococcaceae, which may help both species process and digest sodium and iron.

Even if it’s not the most efficient way to obtain nourishment, when there is no food to go around, vampire finches don’t hold back, attacking both adult boobies and their chicks, reported Matt Simon for Wired in 2014.

"They seem to be purposefully going up to a booby chick in the nest, and they peck at the base of their tail where they have oil glands, and they make it bleed, and they drink the blood," Ken Petren, an evolutionary ecologist at the University of Cincinnati who was not involved in the study, told Wired.

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