Real-life vampire birds hunt and drink blood in remote Ecuador

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. They also saw them in action.

“We observed scores of vampire finches clamouring all over the backs, tails, and wings of boobies, opening up substantial wounds with their sharp beaks, and drinking their fill of blood,” they wrote in an article for The Conversation.

Why only some birds drink blood

“Blood is a notoriously difficult food source to process and extract nutrients from, which probably partly explains why so few animals have evolved to feed on it,” Baldassarre tells Popular Science. He suspects that on these islands where freshwater is extremely rare, blood might serve as a water source for the finches.

These two nazca boobies perform their species' mating call. — Two Nazca boobies perform their species’ mating call. Vampire ground finches feed on Nazca boobies on the remote Galápagos Islands of Wolf and Darwin. *Image: Getty Images /* inthedark72

It’s not the only component of the vampire finches’ diet, however. Based on unpublished data collected by Baldassarre and his colleagues, blood makes up about one-tenth of the vampire finches’ diet.

“These finches are quite opportunistic, so they will eat just about anything they can find,” Baldassarre says. “We observed them feeding on everything from seeds to insects to broken seabird eggs.”

One theory is that vampire finches rely more on blood feeding in periods of extreme drought, when other resources are scarce. But more research is needed to see if this idea holds up in practice.

How this strange behavior evolved

Vampire finches weren’t always vampires, scientists think. The relationship between vampire finches and boobies probably started off with the finches eating parasites found on the skin and feathers of the boobies. This would have been mutually beneficial because it provided a cleaning service for the boobies and food for the finches.

But at some point in evolutionary history, in the process of picking off parasites, the finches started to create lesions on the boobies’ skin, and eventually began feeding on their blood.

Over time, their bills became more scalpel-like, perfect for making skin incisions. Their digestive system also changed. When Baldassarre and his colleagues studied the microbes found in vampire finches’ guts, they found that their gut microbiome was very different from that of other, non-blood-consuming finches on the Galápagos Islands, something they attribute to their unique diet.

Until recently, vampire finches were considered a subspecies of the sharp-beaked ground finch (Geospiza difficilis).

“But they have now been designated as their own species [Geospiza septentrionalis] because of the differences they have evolved because of blood feeding,” Baldassarre notes.

small brown birds eats from flower. — Vampire ground finches on Wolf island have been seen drinking nectar from Galápagos prickly pear flowers and other flowers. *Image: Peter Wilton / CC BY 2.0*

How the boobies cope

Baldassarre and his colleagues have seen vampire finches gang up on and kill nestling boobies,“but that was quite rare,” he points out.

“In most cases, the finches seem to inflict just enough damage to break the skin and extract some blood, but not too much to harm the boobies significantly.” This is similar to many parasite-host relationships, such as fleas, ticks, and mosquitoes on mammals. “You can kind of think of the vampire finches as giant mosquitoes,” Baldassarre says.

As seen in a segment on vampire finches in David Attenborough’s documentary A Perfect Planet, the boobies seem to tolerate the vampire finches rather than chasing them away. There are likely a few reasons for this, Baldassarre says.

“For one, as in large animals being bitten by mosquitoes, often the damage is minimal and not noticeable, so it doesn’t elicit a strong reaction. Also, the boobies sometimes have a vested interest in staying put and guarding their spot in the colony, or perhaps nest or nestlings, so may not want to fly away. Finally, the finches seem to target hard-to-reach areas on the boobies like the base of tail feathers, so it’s likely difficult for the boobies to deter them.”

I, for one, was feeling very sorry for the boobies being constantly hounded by these small vampire birds. Until I learnt that Nazca boobies practice something called “obligate siblicide,” which essentially means that the first-hatched chick routinely attacks and kills its younger sibling. It’s a harsh, bird-eat-bird world in the Galápagos Islands.

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