Creature Feature: Vampire finch

 

We’ve seen cute, seemingly-innocent songbirds turn out to be murderous monsters on this blog before. We won’t be fooled again. So, when I mention Geospiza difficiulus septentrionalis, an unassuming “little brown job” that lives on the Galápagos islands west of Ecuador, you might be hesitant to believe that this species is just another “normal” bird. Well, your distrust would be justified:  this ground finch is a literal vampire.

vampire_finch_1
Don’t let the boring brown look of this finch fool you. [Source]
This bird, the vampire ground finch, is one of Darwin’s famous Galapagos finches.  Yes, these are the same finches that star in Evolution 101 textbook figures, exemplifying the power of natural selection to shape bodily traits and ultimately, form new species**.  As you may recall from those textbook chapters, there are 14 – 18 finch species (species delineations are constantly changing in this well-studied family) spread across the Galápagos archipelago. Their famous beaks have been shaped by selection to suit each species’ particular diet.  The common ancestor of all these diverse finches was likely a seed-eater, and since, the species have beaks adapted to crush seeds, manipulate spiny seed pods, peck at insects in trees, or nibble cactus.

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The Galápagos finch beak radiation matches each species’ diet. [ Source]
Although the beak of the vampire finch isn’t shaped like two little fangs, this bird has the pointiest beak of them all (the better to peck you with, my pretty!). The favored prey of the vampire finch are the booby species that also live on the same two islands as the vampire finches, Wolf and Darwin Island. To feed, the finches “typically jump on the backs of Nazca boobies and pierce a hole near the top of the base of the tail. They will cling to the booby’s feathers for several minutes drinking their blood,” describes Dr. Daniel Baldassarre, an assistant professor at SUNY Oswego who recently studied vampire finches on the islands.  The boobies don’t seem particularly bothered by the fact that they are being fed upon (as seen in the video below). However, the vampire finches aren’t just taking a nibble of booby blood.  Dr. Baldassarre describes being  “shocked by just how much blood they would consume at once. We would catch finches and find their crops bulging with blood. They would often have blood stains all over their bills.”  A songbird with its body gorged with blood, like a mosquito’s after a meal? Talk about disturbing!

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A vampire finch caught red-billed. At right, the crop of a vampire finch bulging with blood after feeding. Photo credit: Daniel Baldassarre

So, where does the vampire finch fit into the whole natural-selection, adaptive-radiation finch story?  Scientists hypothesize that the barren conditions on Wolf and Darwin Islands during the dry seasons pushed these once-normal populations of ground finches to extreme, blood-sucking measures.  Across the Galápagos, the dry season drives all finch species to desperation, forcing them to search for food from unlikely sources, like the spiny pods of Tribulus plants or cacti pads (Grant & Grant 2014). These cycles of harsh environmental pressures may help drive the diverse tactics and traits of the finches observed today. In the case of the vampire finch, it’s proposed that their ground finch ancestors first began feeding on the parasitic flies that swarm on the local booby species (Bowman & Billeb 1965). Later, as the finches became more and more desperate, they found they could break the skin and slurp blood from the boobies’ backs. In fact, the vampire finches’ non-blood-sucking relatives, the sharp-beaked ground finches (Geopsiza difficilus spp.), are still eating insects normally on nearby islands.  The vampire finch remain considered a sub-species of this group, classified separately due to the “behavioral innovations” of blood-sucking they learned in recent evolutionary history (Tebbich et al 2010).

 

Although the vampire finches diverged relatively recently, their disturbing dietary habits may be changing their bodies even further. In a recent study, Dr. Baldassare and colleagues found that the vampire finches have a gut microbiome that differs from other Darwin’s finches (Michel et al 2018). The gut microbiome, which refers to all the little bacterial friends and hitchhikers that live inside the intestines, helps digest nutrients and plays a role in immune response and is heavily influenced by an animals’ diet (giving a whole new meaning to “you are what you eat”!). The vampire finches had communities of bacteria unlike those seen your garden-variety songbird, likely due to their bloody tastes.  Additionally, the research team measured the ratio of nitrogen isotopes in different finch species. Higher up on the food chain, animals accumulate more of one nitrogen isotope than the other, so herbivores tend to have lower levels of this nitrogen form than carnivores. The study found that vampire finches had nitrogen isotope levels on par with marine carnivores like polar bears and sea lions, unlikely company for a little brown bird! This all makes sense, though, when you consider that most of the vampire finches diet is booby blood, and boobies themselves eat fish.

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One of these animals is not like the others…. but all have similar stable isotope signatures!

Overall, the vampire finches are yet another amazing – and slightly scary – example of the power of environmental pressures to drive animals to bizarre behaviors that can ultimately lead to the formation of new species.


**We’ll conveniently ignore the fact that Darwin came up with most of his ideas of natural selection based on his breeding experiments with pigeons, not the finches, but that is a story for another time… 

Acknowledgments: Special thanks to Dr. Daniel Baldassarre for contributing photos and quotes from his field work to this piece!

References

Bowman, R.I. and Billeb, S. I. (1965) Blood-eating in a Galápagos finch. Living Bird 4:29-44.

Grant, P. and Grant, B.R. (2014) 40 Years of Evolution: Darwin’s finches on Daphne Major Island.  Princeton University Press, Princeton, NJ, USA.

Michel, A.J., Ward, L.M, Goffredi, S.K., Dawson, K.S., Baldassarre, D.T., Brenner, A., Gotanda, K.M., McCormack, J.E., Mullin, S.W., O’Neill,A., Tender, G.S., Uy A.C., Yu, K., Orphan, V.J., Chaves, J.A.  (2018) The gut of the finch: uniqueness of the gut microbiome of the Galápagos vampire finch. Microbiome, 6:16

Tebbich, S., Sterelny, K., Teschke, I. (2010) The tale of the finch: adaptive radiation and behavioural flexibility.  Philosophical Transactions of the Royal Society B, 365(1543):1099-1109.

For more information on Darwin’s finches, see

  • The Beak of the Finch by Jonathan Weiner.  A beautifully written, popular science book on Darwin and his finches, and the work of Peter and Rosemary Grant to show the selective processes working on finch populations.
  • 40 Years of Evolution: Darwin’s Finches on Daphne Major Island by Peter and Rosemary Grant. This book gives more scientific details about the body of work done on the Galapagos finches since Darwin.

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