Darwin's finches, also called the Galápagos finches, are a group of about 18 species of songbirds. They are famous for showing how species can change over time to fit different environments, and for having many different types of beaks that help them eat different foods. Scientists often group them as the subfamily Geospizinae or tribe Geospizini. These birds are part of the tanager family and are not closely related to true finches. Their closest living relative is the South American dull-coloured grassquit (Asemospiza obscura). The first Darwin's finches were collected during the second voyage of the Beagle, when Charles Darwin was on the ship as a naturalist. Except for the Cocos finch, which lives on Cocos Island, all other species are found only on the Galápagos Islands.

The term "Darwin's finches" was first used by Percy Lowe in 1936. It became widely known in 1947 when David Lack wrote a book titled Darwin's Finches. Lack studied many bird specimens collected during a scientific expedition to the Galápagos Islands in 1905–06. He dedicated his book to the scientists who led that expedition. Darwin's finches range in size from 10 to 20 cm (4 to 8 in) and weigh between 8 and 38 grams (0.3 to 1.3 oz). The smallest species are the warbler-finches, and the largest is the vegetarian finch. The most noticeable differences between species are their beak shapes and sizes, which help them eat different foods. Food sources on the Galápagos Islands varied between islands and could change suddenly due to events like droughts. All Darwin's finches have dull colors. Scientists believe they evolved from a single finch species that arrived on the islands more than a million years ago.

Darwin's theory

During the survey voyage of the HMS Beagle, Darwin did not realize how important the birds of the Galápagos Islands were. He had learned how to preserve bird specimens from John Edmonstone while studying at the University of Edinburgh. He enjoyed shooting birds but had no training in the study of birds, called ornithology. By the time of the voyage, he focused more on studying rocks and geology. In the Galápagos, he mostly left bird shooting to his servant, Syms Covington. However, these birds later helped Darwin begin his theory of evolution by natural selection.

At the Galápagos Islands and after, Darwin believed in "centres of creation" and did not accept the idea that species could change over time. From his teacher, Henslow, he was interested in how species spread across the world, especially how species on islands compared to those on nearby continents. On Chatham Island, he noticed a mockingbird similar to ones he had seen in Chile. After finding a different mockingbird on Charles Island, he carefully recorded where each mockingbird was caught. He paid less attention to finches. When he examined his bird specimens on the way to Tahiti, he noted that mockingbirds from Charles Island were one species, those from Albemarle were another, and those from James and Chatham Islands were a third. Later, while returning home, this discovery, along with other observations like those about Galápagos tortoises, made him question whether species remained unchanged.

After returning from the voyage, Darwin presented the finches to the Zoological Society of London on January 4, 1837, along with other bird and mammal specimens he had collected. The bird specimens, including the finches, were given to John Gould, a well-known English ornithologist, for identification. Gould paused his other work and, at the next meeting on January 10, reported that the birds Darwin had thought were blackbirds, "gross-beaks," and finches were actually a group of ground finches, making up 12 unique species. This discovery was reported in newspapers.

At the time, Darwin was in Cambridge. In early March, he met Gould again and received a full report about the findings, including the fact that the Galápagos "wren" was another type of finch. The mockingbirds Darwin had labeled by island were separate species, not just variations. Gould found more species than Darwin had expected and concluded that 25 of the 26 land birds were new and distinct forms found only in the Galápagos but closely related to species in South America. Darwin realized that if finch species were limited to individual islands, like the mockingbirds, this could explain the many species on the islands. He sought more information from others on the expedition. Specimens had also been collected by Captain Robert FitzRoy, his steward Harry Fuller, and Darwin’s servant, Syms Covington, who labeled them by island. From these, Darwin tried to determine where he had collected his own specimens. These findings supported his idea that species could change over time.

When Darwin rewrote his diary for publication as Journal and Remarks (later called The Voyage of the Beagle), he described Gould’s findings about the number of bird species, noting that "Although the species are thus peculiar to the archipelago, yet nearly all in their general structure, habits, color of feathers, and even tone of voice, are strictly American." In the first edition of The Voyage of the Beagle, Darwin wrote:

By the time the first edition was published, Darwin was developing his theory of natural selection. In the 1845 second edition of The Voyage (now titled Journal of Researches), Darwin added more details about the beaks of the birds and included two closing sentences that reflected his changing ideas:

Darwin discussed the differences among bird species in the Galápagos more clearly in his chapter on geographical distribution in On the Origin of Species. However, he did not specifically mention the finches in that work.

Polymorphism in Darwin's finches

Darwin visited the Galápagos Islands for five weeks, and David Lack spent three months there. In contrast, Peter and Rosemary Grant and their team have conducted research trips to the Galápagos for about 30 years, focusing especially on Darwin's finches.

Female finches have two different types of songs, called A and B, which are very different from each other. Additionally, male finches with song type A have shorter beaks than males with song type B. These differences in beak size allow the males to eat different parts of their favorite cactus, the prickly pear Opuntia. Males with longer beaks can punch holes in the cactus fruit to eat the fleshy part around the seeds, while males with shorter beaks tear open the base of the cactus to eat the pulp, along with insect larvae and pupae. Both groups also eat flowers and buds. This difference in feeding habits helps them find food more easily during the non-breeding season when food is limited.

If the finch population is panmictic, meaning all members can mate freely with each other, the species Geospiza conirostris shows a balanced genetic variation instead of being an early stage of sympatric speciation, as previously thought. The genetic variation helps the species use its environment more effectively by increasing its feeding options. Without a detailed breeding study, the genetic reasons for this variation are unclear, but two genes that work closely together may be involved.

Another difference is the color of the beaks in young finches, which can be either pink or yellow. All species of Darwin's finches show this trait, which lasts for about two months. The cause of this difference is not yet understood.

Taxonomy

For many years, scientists called taxonomists have grouped these birds with the New World sparrows and Old World buntings in the family Emberizidae. However, the Sibley–Ahlquist taxonomy places Darwin's finches with the tanagers (Monroe and Sibley 1993), and one recent study agrees with this classification (Burns and Skutch 2003). The American Ornithologists' Union includes the Cocos finch in the Emberizidae family but adds an asterisk to show the placement might be incorrect (AOU 1998–2006). In its South American checklist, the Galápagos finch species are listed as incertae sedis, meaning their classification is uncertain (Remsen et al. 2007).

• Genus Geospiza
• Genovesa ground finch (Geospiza acutirostris)
• Española cactus finch (Geospiza conirostris)
• Sharp-beaked ground finch (Geospiza difficilis)
• Vampire finch (Geospiza septentrionalis)
• Medium ground finch (Geospiza fortis)
• Genovesa cactus finch (Geospiza propinqua)
• Small ground finch (Geospiza fuliginosa)
• Large ground finch (Geospiza magnirostris)
• Common cactus finch (Geospiza scandens)

Big Bird (not yet formally named): In 1981, a hybrid male arrived on Daphne Major island. It mated with local Galápagos finches (G. fortis) and created a new population of larger birds that can eat different foods. These birds do not breed with other species on the island because females do not recognize the males’ songs. Genetic studies show that this population is now completely isolated from native species, meaning it cannot breed with them. Professor Leif Andersson of Uppsala University says a taxonomist who does not know this bird’s history would likely classify it as a separate species.

• Genus Camarhynchus
• Large tree finch (Camarhynchus psittacula)
• Medium tree finch (Camarhynchus pauper)
• Small tree finch (Camarhynchus parvulus)
• Woodpecker finch (Camarhynchus pallidus) – sometimes placed in Cactospiza
• Mangrove finch (Camarhynchus heliobates)

• Genus Certhidea
• Green warbler-finch (Certhidea olivacea)
• Grey warbler-finch (Certhidea fusca)

• Genus Pinaroloxias
• Cocos finch (Pinaroloxias inornata)

• Genus Platyspiza
• Vegetarian finch (Platyspiza crassirostris)

Modern research

A long-term study conducted over 40 years by Princeton University researchers Peter and Rosemary Grant has shown how changes in the size of finch beaks are influenced by El Niño and La Niña weather patterns in the Pacific Ocean.

In 2004, scientists discovered that a protein called bone morphogenetic protein 4 (BMP4) plays a role in shaping finch beaks. BMP4 helps form the skeletal structure of the beak, making it stronger. Researchers also found that the timing and location of activity for a gene called calmodulin (CaM) during development affect the shape of the beak, such as making it longer or more pointed. These findings suggest that changes in how BMP4 and CaM work during growth may control how beaks develop. Additionally, these changes in beak size have also affected the sounds that finches make.

Between 2015 and 2016, genome studies identified a specific set of genetic instructions near the ALX1 gene, which is involved in the development of the skull and face. This genetic pattern is strongly linked to the variety of beak shapes seen in finches. A blunt beak is a feature that evolved later. This genetic pattern includes two specific changes in the DNA code: L110P and I209V.

In 2016, scientists sequenced the genomes of all Darwin’s finch species and found that a single small change in the DNA code of a gene called HMGA2 is strongly connected to differences in beak size. HMGA2 is a gene that, in humans, is linked to variations in height, skull shape, and tooth development.

A study of the genomes of three Geospiza ground finch species living in the same area found that 11 out of 32,569 small DNA changes (SNPs) were linked to four groups of genetic markers. These groups together explained 83.6% of the differences in beak size among the finches. This means that only a small part of the finches’ DNA is responsible for changes in beak shape, which helps explain how beak forms can change quickly in response to different environments on the Galapagos Islands.

Phylogeny

Scientists have created many family trees for Darwin's finches using DNA and physical traits. Some of these trees group the birds differently than what is currently accepted, which may suggest problems with how species are classified today. Some studies, like Sato's DNA research, show that the birds are so similar that all Geospiza might be one species with many forms. The same idea has been proposed for Camarhynchus on Floreana Island.

A family tree from Lamichhaney et al. (2018) uses whole-genome data, which is more accurate than DNA from mitochondria. This data clearly shows differences between populations.

• C. fusca (San Cristobal)
• C. olivacea (Santiago)
• P. crassirostris (Santa Cruz)
• G. difficilis (Fernandina)
• G. difficilis (Santiago)
• G. difficilis (Pinta)
• outgroup (T. bicolor, L. noctis)

However, a family tree cannot show the mixing of genes within Geospiza. For details about gene mixing, refer to Lamichhaney et al. (2015), Almen et al. (2016), and Lamichhaney et al. (2018). A gene called ALX1, which affects beak shape, is often shared between species. Two species now separated from G. difficilis, though not closely related to G. difficilis in whole-genome studies, share many similar genetic traits. This suggests they may have inherited their similar features through ancient gene mixing.