January 27, 2021 | Jerry Bergman

Why Does This Finch Drink Blood?

Did Some of Darwin’s Finches Evolve to Drink Blood?
An Evaluation of this Claim

by Jerry Bergman, PhD


The diet of most vertebrates tends to be specialized, but flexible in extreme circumstances. Humans decidedly prefer certain foods, but in extreme circumstances will sometimes consume almost anything, even urine and other humans. Some birds consume primarily seeds, others worms, yet others nectar or sugar water. Robins prefer worms and insects but, if they are unable to find worms, will consume other foods, like fruit, raisins, suet, berries, and seeds.[1] Likewise, some finches favor seeds, others prefer flower nectar, pollen and insects.

Darwinists believe that all food preferences evolved, so why did the discovery that some finches consume blood recently merit headlines? The reason is (it is implied), that, historically, finches did not include blood in their diet but, in extreme circumstances, they recently evolved the ability to eat blood. Thus, evolution is occurring in front of our very eyes! The claim is “Scientists suggest the vampire finch evolved to drink blood to survive the volcanic archipelago’s harsh environment and scarce resources.”[2]

Figure 1. Darwin’s Finches. Sketches made by Darwin. From Wiki Commons.

The Jan 22, 2021 article by Elizabeth Gamillo coming out of the Smithsonian repeats the oft-told story that 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.”[3]  This includes the vampire finches. Darwin observed the differences in the finches’ diet on various islands and later observed their beak size changed as island isolation forced the finches to adapt to available food resources. The island’s environment is extremely harsh and certain major food sources disappear during the dry season, forcing them to change their diet.[4] Finches on islands where there is a shortage of seeds, but many grubs living under tree bark, will switch to consuming grubs. In a finch population with much variation, some finches will have longer beaks and some shorter beaks than average. Those birds carrying more of the ‘long-beak’ genetic information are more likely to survive on grubs because they need a longer beak to reach them in their habitat. Thus, they would be more likely to pass this genetic information on to their descendants, and concurrently the ones with short beaks would be more likely to die out.

Princeton Professors Peter and Rosemary Grant

We have an advantage in evaluating this finch evolution claim because the Galápagos Island finches have been the subject of one of the most detailed studies ever completed on natural selection. The research by Princeton professors Peter and Rosemary Grant involved years of field observation in the Galápagos Islands. Their focus was on researching the changes in the finch population, and evaluating the effect of changing climatic conditions, which dramatically altered their food supply.

Figure 2. Note the beak differences and how tiny they are. From Wiki Commons.

They found that finches have thrived or died, depending on which species’ beak structure was best adapted for the most abundant food at a given time. This kind of variation is seen in most life-forms. The birds were not evolving, but certain traits became more common when some food sources were abundant, and became less common when these food sources became less abundant, for example by a drought. When the drought ended, the seeds became more abundant, and the specific birds that fed on them once again became more common. Only the number of individual birds within each type having a certain beak size was fluctuating, increasing and decreasing according to the food supply. No evolution was taking place.

Some Details of Finches

Finches are tiny birds, only 125 to 140 mm long, about 5 to 5.5 inches. Their tiny beaks vary in length, width, and depth due to genetic-controlled molecular mechanisms. Specifically, the Bmp4 gene was correlated with finch beak depth and width.[5] Furthermore, calmodulin expression controlled by genes is correlated with greater beak length.[6] In addition, the premaxillary bone contributes to the final beak shape. Lastly, differences in the expression of TGFβIIr, β-catenin, and dickkopf-3 (DKK3) genes affect beak-shape differences.

The differences that the Grants studied were not caused by mutations, but by genes already built into the birds. Even though the Grants’ research was commonly billed as proving evolution, it did nothing of the sort. The finches’ inbuilt genetic design provides for beak adjustments to enable the birds to thrive in a wide variety of habitats. Nor do the beak changes provide evidence that the birds evolved from beakless ancestors. That is because all of the genes described above constitute a set required to produce a beak. Without a beak a finch could not eat! Lacking any one of these genes would produce beakless birds that could not survive. This illustrates Michael Behe’s principle about  irreducible complexity: either every part of a functioning whole must be present and working or nothing works.

Birds expressing certain traits which allow them to thrive in their environment would become more numerous, producing the variations in population numbers. These details reveal a purposeful design. In short, from the facts and arguments, “it appears to be clear that no macroevolution is happening in “Darwin’s finches” on the Galápagos Islands,” says German biologist Wolf-Ekkehard Lönnig. [7]

The reason this topic is important is due to Darwin’s legacy in the media. The Galapagos is the most widely-known stopover made by Charles Darwin during his five-year voyage on The Beagle. The five weeks he spent there in September 1835 “would change the world of science … Among other finds, he observed and collected the variety of small birds that inhabited the islands… It was not until he was back in London, puzzling over the birds, that the realization that they were all different, but closely related, species of finch led him toward formulating the principle of natural selection.”[8] In the 20th century up to today, the finch beak differences on different islands has been touted as a major evidence of evolution. As stated above, this claimed “evolution in action,” which produced “an iconic model for studies of speciation and adaptive evolution” proved nothing of the sort.[9] If anything, it showed that irreducible complexity has a slight bit of wiggle room.

The Evolution of the Blood-Sucking Finch

Figure 3. Vampire finches will resort to drinking blood for survival when they can’t find other food sources like seeds and insects. From Wiki Commons.

On Darwin and Wolf Islands lives one type of blood-sucking finch, the Geospiza septentrionalis, which “uses its razor-sharp beak to pierce the wings of a large sea bird called the Nazca booby, Sula granti, and drinks its blood.”[10]  While a peculiar behavior for sure, it turns out to have nothing to do with evolution, but rather with learning and experience. Gamilo’s article explains the vampire finches first consumed 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.”[11]

Assuming this scenario is correct, they learned this behavior in desperation looking for food. The next question was “can they survive on blood”? It may satisfy their hunger needs for the moment, but can it sustain them nutritionally?

Blood is Not an Ideal Diet for Fiches

Blood is low in certain necessary nutrients and high in iron and sodium chloride (table salt). Both vampire finches and the vampire bats contain a common gut bacteria Peptostreptococcaceae which helps both species process and digest iron and sodium from the poisonous salt.[12] Only when food is largely unavailable will vampire finches attack adult boobies and their chicks. To make it bleed they often peck at the base of their tail where their oil glands are located. Then they drink their blood.[13] No evidence of evolution was observed. How the vampire bats became infected with Peptostreptococcaceae is likely because one or a few finches were infected with this common gut bacteria. Those so infected were far more likely to survive and have offspring, thus the number of finches infected soon included the entire population. The rest died off during the dry season.


The title of the Smithsonian Magazine article was “Why Some of Darwin’s Finches Evolved to Drink Blood,” yet no evidence of its evolution was cited. In spite of arguments that this example somehow proved or at least documented evolution, no evidence of the evolution of the vampire trait existed. The trait was purely a result of insect-eating birds discovering that, when their food supply was significantly low, consuming blood allowed them to avoid starvation. Fortunately, they contained a gut bacterium that allowed them to consume this source of food to survive. Likely, this same bacteria, found in other animals such as other birds and even cats, allowed them to consume insects and, later, blood.

Figure 4. Some finches are incredibly beautiful birds. Note again how small the beak is compared to the 5inch-long body. From Wiki Commons.



[1] Wheelwright, Nathaniel T. The Diet of American Robins: An Analysis of U.S. Biological Survey Records, The Auk 103(4):710-725, October 1986, https://www.jstor.org/stable/4087182?seq=1;  Hoskins, Rachel. What do robins eat? Woodland Trust, 1 December 2019,


[2] Gamillo,  Elizabeth. Why some of Darwin’s finches evolved to drink blood, Smart News, 22 January 2021, https://www.smithsonianmag.com/smart-news/why-some-darwins-finches-evolved-drink-blood-180976814/

[3] Gamillo, 2021.

[4] Gamillo, 2021.

[5] Abzhanov, A., Protas, M., Grant, B.R., Grant, P.R. and Tabin, C.J., Bmp4 and morphological variation of beaks in Darwin’s finches, Science 305(5689):1462–1465, 3 September 2004.

[6] Abzhanov, A., Kuo, W.P., Hartmann, C., Grant, B.R., Grant, P.R. and Tabin C.J., The calmodulin pathway and evolution of elongated beak morphology in Darwin’s finches, Nature 442(7102):563–567, September 2006.

[7] Lönnig, Wolf-Ekkehard. 2020. “Darwin’s finches”: Galápagos Islands as an evolutionary model, Evolution News & Science Today, 30 November 2020,  https://evolutionnews.org/2020/11/darwins-finches-galapagos-islands-as-an-evolutionary-model/.

[8] “Adaptive Radiation: Darwin’s Finches”, PBS Evolution Library, https://www.pbs.org/wgbh/evolution/library/01/6/l_016_02.html

[9] Lamichhaney, S., et al., angeet 2015. Evolution of Darwin’s finches and their beaks revealed by genome sequencing, Nature 518:371-375, 11 February 2015, https://www.nature.com/articles/nature14181.

[10] Sawal, Ibrahim. 2021. A perfect planet review: Attenborough’s new show is one of his best, New Scientist,

[11] Gamillo, 2021.

[12] Michel, A., et al., The gut of the finch: uniqueness of the gut microbiome of the Galápagos vampire finch, Microbiome 6:167, 19 September 2018, https://microbiomejournal.biomedcentral.com/articles/10.1186/s40168-018-0555-8

[13] Simon, M., Absurd creature of the week: The tiny blood-slurping bird that terrorizes the Galapagos, Wired, 4 July 2014, https://www.wired.com/2014/07/absurd-creature-of-the-week-vampire-bird/

Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,300 publications in 12 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,500 college libraries in 27 countries. So far over 80,000 copies of the 40 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.

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