For most people, the word “vampire” is reminiscent of Dracula, or perhaps hunters like Blade or Buffy; or maybe even the vampire bats of South America. Few will think of one small and rather beautiful bird – the finch.
But there are indeed “vampire finches” that feed on the blood of much larger birds and they have been introduced to the world in a fantastic segment of Perfect Planet, the new series directed by David Attenborough for the BBC. For us, these finches did not need an introduction as we examined them carefully.
These birds are found in the Galapagos Islands, a volcanic archipelago located about 1,000 km off the coast of Ecuador. The islands are a biodiversity hotspot partly because of their isolation. Organisms that somehow make it to the Galapagos Islands have to adapt to the harsh conditions or become extinct.
One such group of organisms are the Darwin finches. Named after the naturalist Charles Darwin, who gathered examples on his famous voyage aboard the HMS Beagle, this group of finches is made up of several species that evolved from a common ancestor. Each species has developed a different beak size and shape, which allows it to utilize different foods. For example, the cactus finch has a long, thin string that enables it to consume the nectar from cactus flowers. Some species have bills that are better at chopping up seeds, while others are better at consuming insects or plants.
How the bloodsucking developed
The vampire finches are only found on Wolf and Darwin, the two northernmost islands of the archipelago, and are even remote by Galapagos standards. Both islands are tiny, less than a square mile each, and separated from the larger islands by 100 miles of open ocean. Fresh water is extremely rare and some foods can disappear completely during the dry season.
Nazca Booby, Copyright Dave Williamson, from the Surfbirds Galleries
At some point in the last half a million years – evolutionarily new – finches arrived on Wolf and Darwin and began to coexist with large seabirds that nest on the islands, such as red-footed and Nazca boobies. Over time, the finches appear to have evolved to eat parasites found in the feathers and on the skin of the boobies. This was “reciprocity” in action: the boobies benefited from the removal of parasites, and the finches benefited from an alternative to their usual diet of nectar, seeds and insects that can disappear during the dry season.
Ultimately, however, the removal of parasites resulted in open skin lesions on the boobies, which allowed the finches to consume blood. The finches even learned to pierce the skin at the base of young feathers to access the blood directly, eliminating the need for the insect parasites. So the finches used an alternative food source, blood from the fools, and earned the nickname “vampire finches”.
It’s hard to know exactly how much of the finch’s diet is made up of explosive blood, but our unpublished data suggest that it is one-tenth. The natural selection seems to have adjusted the vampire finch beak to skin penetration and blood sucking, as the birds have developed particularly long and pointed beaks compared to non-blood-feeding populations on other islands. And once a blood donor pierces the skin, they still need a way to consume and digest the blood. When we examined the microbes found in the intestines of these vampire finches in search of adaptations, we found a very different microbiome than any other species of Darwin’s finch, believed to be caused by blood nutrition.
What it’s like to see in person
Two of us, Daniel and Jaime, went with Professor Albert Uy to Darwin and Wolf to study these fascinating finches on islands that are rarely visited even by researchers. Getting there was extremely difficult as there are no beaches for a boat to land. We had to use a small dinghy to approach the cliffs and then wait for a short gap in the waves before jumping on sharp, black lava stones. But this isolation means vampire finches are plentiful, and the dense breeding colonies of gannets made it easy to imagine how this strange blood-sucking behavior might have evolved.
The boobies are incredibly vulnerable when looking after nests and chicks, as they are reluctant to leave them temporarily themselves. We watched dozens of vampire finches howling over the back, tails and wings of boobies, opening substantial wounds with their sharp beaks, and drinking their blood. Interestingly, the finches appear to act like a real parasite, causing enough damage to secure a meal without unduly harming the host.
For the boobies, the whole experience is really very much like a human being attacked by mosquitos. Although they can tolerate the finches, the tiny bloodsuckers are a nuisance that the fools want to get rid of. And if it all gets too much, they can be forced to fly away.
And who can blame them? When we caught finches to collect samples and found tubes full of blood and red beaks. It was obvious that the little vampires were not just loosening a few drops of blood.