Darwin's Finches

The Galápagos finch species collectively form a showcase example of Charles Darwin's theory of natural selection. The 12 or 13 species of Galapagos finches are often called "Darwin's Finches". They are used as an example of how the descendants of one ancestor can evolve into several species as they adapt to different conditions.


These drab, quite dull birds are linked forever with Darwin's name, yet he wasn't the first to see them (Captain James Colnett, 1798 may have been) and he didn't write much about them in his book on 'The Voyage Of The Beagle'. Nevertheless they featured later in his life as he recognised the evolutionary process. Their name was popularized by the ornithologist David Lack in 1947, and since Darwin's Finches have been the subject of major studies, revealing much about the evolutionary process.

There are 14 species of birds recognised as Darwin's Finches - 13 in the Galapagos, and one on Cocos Island (Costa Rica). Whilst they are all of roughly similar size, colour their difference is in the size and shape of their beak and allows a different niche to be occupied. Warning - beak shapes are variable, size and shape in one can overlap into the range of another species - hence identification is really hard.

The rest of this section I have lifted as a quote from a great source on The Galapagos - Dr. Robert Rothman, Professor, Rochester Institute of Technology, College of Science ... see http://people.rit.edu/rhrsbi/HomePage/background.html.

(...) why we need to worry about naming them; why can't we just enjoy them for what they are? There is power in a name; to know the name is to understand the named. This is especially so in that branch of biology known as taxonomy or systematics. The taxonomist not only applies a name to an organism, but, by ranking those organisms into hierarchies of names, attempts to portray evolutionary relationships. Since 1758, taxonomists have used the system of Linnaeus to organize the living world. Linnaeus gave each organism a binomial: genus and species, which are ranked in higher and higher groupings:

Kingdom

a group of related phyla

Phylum

a group of related classes

Class

a group of related families

Order

a group of related genera

Genus*

a group of related species

species**

an individual type of organism

It should be clear from this table that the only "real" entity is the species. The higher groupings are merely an assessment of how species are thought to be related to other species, and different taxonomists may very well disagree. Among the Darwin's finches, there is general agreement as to the existence of 13 Galapagos species, although there may be one or two more or one or two less, depending on how one assesses several unusual populations. There is disagreement, however, about how those 13 species are organized into genera. Traditionally, the finches are divided into four groups, each representing a single genus: the ground finches (Geospiza), the tree finches (Camarhynchus), the warbler finch (Certhidea) and the Cocos finch (Pinaroloxias). As a group, the tree finches are more heterogeneous than the ground finches and it is current practice to subdivide the tree finches into three genera: Camarhynchus (the tree finches), Platyspiza (the vegetarian finch) and Cactospiza (the woodpecker and mangrove finches). On the other hand, finch expert David Steadman feels that splitting the finches into six genera emphasizes their differences and suggests that all of the finches should be united as 14 species in the singe genus Geospiza to emphasize their similarities!! But whether you split them into six genera or lump them into one, everybody pretty much agrees on the same 14 species. The only real entity is the species. The table below gives the genus and species names for all of the finches:

THE FOURTEEN SPECIES OF DARWIN'S FINCHES

A. Ground Finches

1/ Small Ground Finch - Geospiza fuliginosa

2/ Medium Ground Finch - Geospiza fortis

3/ Large Ground Finch - Geospiza magnirostris

4/ Sharp-beaked ground Finch - Geospiza difficilis

5/ Cactus Ground Finch - Geospiza scandens

6/ Large Cactus Ground Finch - Geospiza conirostris

B. Tree Finches

1/ Small Tree Finch - Camarhynchus parvulus

2/ Medium Tree Finch - Camarhynchus pauper

3/ Large Tree Finch - Camarhynchus psittacula

C. Woodpecker Finch - Cactospiza pallidus

D. Mangrove Finch - Cactospiza heliobates

E. Vegetarian Finch - Platyspiza crassirostris

F. Warbler Finch - Certhidea olivacea

G. Cocos Island Finch - Pinaroloxias inornata

Identification of finches can be extremely intimidating.

Dr. Robert Rothman doesn't mention the Vampire Finch, Geospiza difficilis septentrionalis,a close relative of the Sharp-beaked ground Finch - Geospiza difficilis, and I don't know why (but will ask him!).

Now interestingly, if we focus on just one finch - let's pick the large cactus finch Geospiza conirostrison Isla Genovesa (Tower Island), we can find various kinds of morphism. Males are dimorphic in song type: songs A and B are quite distinct. Also, males with song A have shorter bills than B males. With these beaks males are able to feed differently on their favourite cactus, the prickly pear Opuntia echios. Those with long beaks are able to punch holes in the cactus fruit and eat the fleshy aril pulp which surrounds the seeds, whereas those with shorter beaks tear apart the cactus base and eat the pulp and any insect larvae and pupae (both groups eat flowers and buds). This dimorphism clearly maximises their feeding opportunities during the non-breeding season when food is scarce. If the population is panmixic, then Geospiza conirostris exhibits a balanced genetic polymorphism and not, as originally supposed, a case of nascent sympatric speciation. The selection maintaining the polymorphism maximises the species' niche by expanding its feeding opportunity. The genetics of this situation cannot be clarified in the absence of a detailed breeding program, but two loci with linkage disequilibrium is a possibility. Another interesting dimorphism is for the bills of young finches, which are either 'pink' or 'yellow'. All species of Darwin's finches exhibit this morphism, which lasts for two months. No interpretation of this phenomenon is known.

The Large Cactus Finch (Geospiza conirostris) is a species of bird in the tanager family Thraupidae - restricted to Española, Genovesa, Darwin, and Wolf Islands. This rather dark bird resembles the smaller and finer-beaked Common Cactus Finch, but the two species do not co-inhabit any island. There are significant differences between the subspecies of the Large Cactus Finch; G. c. conirostris of Española has a far larger beak than G. c. darwinii and G. c. propinqua of the remaining three islands. Its natural habitat is dry shrub-land and it is commonly seen on the ground. Its main food source is the cactus Opuntia sp.

The Sharp-beaked Ground Finch (Geospiza difficilis) is a species of bird in the Darwin's finch group of the tanager family Thraupidae. This rather small, slender-billed bird is endemic to the Galápagos Islands, where found on Fernandina, Santiago, Pinta, Genovesa, Darwin and Wolf. On the first three islands, it breeds in the humid highlands and disperses afterwards, but on the remaining smaller and lower islands the Sharp-beaked Ground Finch is found in the arid zone year-round. Formerly it was also present in the highlands of several other islands, and it is possible it still occurs on Isabela. The subspecies on the islands of Darwin and Wolf has gained a level of notoriety due to its habit of feeding on blood of large birds, notably boobies, gaining it the common name Vampire Finch (Geospiza difficilis septentrionalis), it's a small bird native to the Galápagos Islands and a distinct subspecies of the Sharp-beaked Ground Finch (Geospiza difficilis) endemic to Wolf and Darwin Islands (Grant et al. 2000). The vampire finch is sexually dimorphic as typical for its genus, with the males being primarily black and the females grey with brown streaks. It has the largest and most pointed beak of all G. difficilis subspecies, and overall looks like a miniature Common Cactus-Finch rather than, as the other subspecies do, a large Small Ground-Finch with a straight bill (Schluter & Grant 1984). It has a lilting song on Wolf, a buzzing song on Darwin, and whistling calls on both islands; only on Wolf, a drawn-out, buzzing call is also uttered.(Grant et al. 2000). This bird is most famous for its unusual diet. The Vampire Finch occasionally feeds by drinking the blood of other birds, chiefly the Nazca and Blue-footed Boobies, pecking at their skin with their sharp beaks until blood is drawn (Schluter & Grant 1984). Curiously, the boobies do not offer much resistance against this. It has been theorized that this behavior evolved from the pecking behavior that the finch used to clean parasites from the plumage of the booby. The finches also feed on eggs, stealing them just after they are laid and rolling them (by pushing with their legs and using their beak as a pivot) into rocks until they break. More conventionally for birds, but still unusual among Geospiza, they also take nectar from Galápagos prickly pear (Opuntia echios var. gigantea) flowers at least on Wolf (Schluter & Grant 1984). The reasons for these peculiar feeding habits are the lack of freshwater on these birds' home islands. Nonetheless, the mainstay of their diet is made up from seeds and invertebrates as in their congeners (Schluter & Grant 1984). The vampire finch is endangered, being a small-island endemic.

Medium Tree Finch - Camarhynchus pauper Critically Endangered as it has a very small range on a single island, and recent information suggests that it is declining rapidly owing to the effects of the parasite Philornis downsi; a species of fly - Diptera Muscidae first recorded in Trinidad, accidentally introduced to the Galapagos and, though the adults feed on fruit, they lay their eggs in nests and the hatchlings are parasitic larvae feeding on developing nestlings thus causing significant mortality.

The Small Ground Finch (Geospiza fuliginosa) is a species of bird in the Darwin's finch group of the tanager family Thraupidae and is common and widespread in shrub-land, woodland and other habitats on most islands in the archipelago. It commonly feeds on parasites from the skin of Galápagos tortoises, and Galápagos land and marine iguanas.

Large Ground Finch (Geospiza magnirostris) of the family Thraupidae, (formerly in the Emberizidae) - with a natural habitat that is subtropical dry shrub-land this finch has a beak that can crack hard shells.

The Common Cactus Finch or Small Cactus Finch (Geospiza scandens) is found on most islands, with the notable exception of Fernandina, Española, Genovesa, Darwin and Wolf. Most of these islands are inhabited by its close relative, the Large Cactus Finch. Its natural habitats are dry scrub-land and woodland. It is usually closely associated with the cactus Opuntia sp.

The Vegetarian Finch (Platyspiza crassirostris) is in the tanager family Thraupidae, monotypic within the genus Platyspiza and is endemic to the Galapagos Islands and its natural habitats are subtropical or tropical dry forests and subtropical or tropical moist montanes (see note after Large Tree Finch write up).

The Large Tree Finch's (Camarhynchus psittacula)natural habitats are subtropical or tropical dry forests and subtropical or tropical moist montane, which are regions generally with cooler temperatures and often have higher rainfall than the adjacent lowland regions, and are frequently home to distinct communities of plants and animals. Note: the term "montane" means "of the mountains". However, montane ecosystems are typically well-forested as they lie considerably below the tree line. The zone around the tree line, where individual trees and krummholz but no forests are found, is known as sub-alpine, while the lowest of the tree-less regions further up a mountain is the alpine zone.

Medium Tree Finch, Camarhynchus pauper is a critically endangered species found on Floreana Island at elevations above 250 metres in moist highland forest habitat its breeding habitat is dominated by Scalesia pedunculata trees but is threatened by habitat loss from clearance for agriculture and introduced predators such as rats, mice, cats, and the Smooth-billed Ani. The diurnal Galapagos Short-eared Owl is its only remaining natural predator. The introduced parasite Philornis downsi is also a significant threat to the survival of this species. Parasitic larvae of this fly live in the nest material and feed on the blood and body tissues of nestlings.P. downsi causes high nestling mortality. They generally lay 2-3 eggs. Eggs are incubated for approximately 12 days, and nestlings are fed by both parents at the nest for approximately 14 days before fledging. The range of beak sizes of the Medium Tree Finch on Floreana and the Large Tree Finch, Camarhynchus psittacula, on Isabela is roughly the same. This reflects the fact that the two species feed on the same type and size of insect. The beak of the Medium Tree Finch is intermediate in size between the Small Tree Finch and Large Tree Finch. All three species are found in the same habitat.




The Small Tree Finch (Camarhynchus parvulus) of the Darwin's finch group of the tanager family Thraupidae, [see picture left].

It is endemic to the Galapagos Islands. Its natural habitats are subtropical or tropical dry forests and subtropical or tropical dry shrub-land.

Woodpecker Finch, Camarhynchus pallidus, [below], occurs widely in the Galapagos Islands, from sea level to high elevations. It has gained fame due to its capability of using a twig, stick, or cactus spine as a tool, which is used as compensation for its short tongue. The finch manipulates the tool to dislodge invertebrate prey such as grubs from trees. The same tool can be used many times on many different trees.

Woodpecker Finch
Scientists have observed that the finches may shorten the stick or spine to make it more manageable. The finches may also try various sticks or spines at one site before finding just the appropriate one that can reach and extract the prey item. During the dry season, Woodpecker Finches use tools during half their foraging time, using these tools to acquire up to 50% of their prey. This means they obtain even more food via tool use than chimpanzees, the most proficient non-human primate tool users.

The Mangrove Finch, (Camarhynchus heliobates)
Poised on the brink of extinction, this diminutive finch is a species of bird in the Darwin's finch group of the tanager family Thraupidae, and is not only the most endangered of Darwin’s finches, but is also one of the rarest birds worldwide. Like the other twelve species of Darwin’s finches endemic to the Galapagos Islands, this species has evolved a specialised beak shape, enabling it to exploit a particular habitat and diet. In the case of the mangrove finch, the beak is long and pointed, with a down-curved culmen, enabling this species to lift scales of tree bark, and pick off insect prey. Plumage is dull brownish above, becoming olive on the rump, while the underparts are whitish with faint streaking. Males can be distinguished by the black colouration that develops on the head and neck over the course of several annual moults. It used to be found on the islands of Fernandina and Isabela, but recent surveys have failed to record the species on Fernandina. It has been classified as critically endangered by BirdLife International, with an estimated population of between 60 and 140 located in two large mangroves on Isabela. A study has shown that the two small populations remaining on Isabela Island have begun undergoing speciation and that one or both populations will eventually become extinct due to a lack of interbreeding. As its name suggests, the Mangrove Finch lives in the mangroves of the Galápagos Islands and feeds upon the various insects, larvae, spiders, and vegetable matter found in the mangroves. It closely resembles the far commoner Woodpecker Finch, but is not known to utilize tools.

The Warbler Finch (Certhidea olivacea) is sometimes classified in the family Emberizidae, but more recent studies have shown it to belong in the tanager family. It is the only member of the genus Certhidea. This species has two distinct subspecies groups, sometimes recognised as separate species. These groups differ in appearance, distribution, habitat, and song. The Green Warbler Finch group consists of only one subspecies, the nominate olivacea, from Santiago, Rábida, Pinzón, Isabela, Fernandina, and Santa Cruz. Green Warbler Finches have a greenish coloration to blend into their lusher semi-humid forest habitats, as well as distinctive reddish throat patches on breeding males. The Grey Warbler Finch group consists of the subspecies fusca from Pinta and Marchena, becki from Darwin and Wolf, mentalis from Genovesa, bifasciata from Santa Fé, cenerascens from Española, luteola from San Cristóbal, and ridgwayi from Floreana. Grey Warbler Finches are found mostly in the shrub-land and dry forest of smaller drier islands, and have a suitable coloration for their habitat. Natural habitats are subtropical or tropical dry forests, subtropical or tropical moist montanes, and subtropical or tropical dry shrub-land.


The Cocos Island Finch or Cocos Finch, Pinaroloxias inornata , is the only one of Darwin's finches not native to the Galápagos Islands, and the only member of the genus Pinaroloxias. Sometimes classified in the family Emberizidae, more recent studies have shown it to belong in the tanager family Thraupidae. It is endemic to Cocos Island, which is approximately 360 miles south of Costa Rica, where it is the most common landbird. It is a chunky 12 cm long finch weighing about 12.5 grammes and with a black decurved pointed bill. The male is entirely black, while the female is brown, which is paler below and heavily streaked. The young are similar but have yellow bills. The roughly spherical nest is built at the end of a branch, and the standard clutch is two brown-spotted white eggs. The Cocos Island Finch is found in all available habitats and eats a wide range of plant and insect food. This species qualifies as Vulnerable because it has a very small range.