Darwin's Finches
Darwin's Finches are confusing.
Their taxonomic position is uncertain. For several decades they have been placed in Family: Emberizidae with the buntings and new world sparrows; more recently it has been proposed they be placed in Family: Thraupidae amongst the tanagers. They are not finches (Family: Fringillidae)
The most important differences between the species are in the size and shape of the bill.
Some species can and do hybridise with other species, and the resultant intermediate forms add to the difficulty of specific identification. Some individuals cannot be reliably determined.
Bill size and shape within species continues to vary and evolve - this is particularly evident in el niño years when the usual food sources may be less available.
Bills of younger birds are pink or yellow; they darken to black with age in all species - so bill colour is not a useful identification feature.
Some populations within a species are dimorphic - one group having longer bills than the other and the males of one group having a different song to the other - it has been observed, in Española Cactus Finch, that this permits the population as a whole to utilise a wider range of food sources. Studies suggest that this dimorphism is balanced and polymixic (males of both morphs can breed with females of both morphs) rather than divergent nascent speciation.
Darwin's finches are placed in 5 genera. One of these (Pinaroloxias) occurs only on the Cocos Islands (near Costa Rica); the other 4 occur only on the Galapagos Islands. Certhidea (Warbler Finches) and Platyspiza (Vegetarian Finch) are relatively easy to identify, I will deal with these before moving on the the more tricky Camarhynchys (Tree finches) and Geospiza (Ground Finches).
Their taxonomic position is uncertain. For several decades they have been placed in Family: Emberizidae with the buntings and new world sparrows; more recently it has been proposed they be placed in Family: Thraupidae amongst the tanagers. They are not finches (Family: Fringillidae)
The most important differences between the species are in the size and shape of the bill.
Some species can and do hybridise with other species, and the resultant intermediate forms add to the difficulty of specific identification. Some individuals cannot be reliably determined.
Bill size and shape within species continues to vary and evolve - this is particularly evident in el niño years when the usual food sources may be less available.
Bills of younger birds are pink or yellow; they darken to black with age in all species - so bill colour is not a useful identification feature.
Some populations within a species are dimorphic - one group having longer bills than the other and the males of one group having a different song to the other - it has been observed, in Española Cactus Finch, that this permits the population as a whole to utilise a wider range of food sources. Studies suggest that this dimorphism is balanced and polymixic (males of both morphs can breed with females of both morphs) rather than divergent nascent speciation.
Darwin's finches are placed in 5 genera. One of these (Pinaroloxias) occurs only on the Cocos Islands (near Costa Rica); the other 4 occur only on the Galapagos Islands. Certhidea (Warbler Finches) and Platyspiza (Vegetarian Finch) are relatively easy to identify, I will deal with these before moving on the the more tricky Camarhynchys (Tree finches) and Geospiza (Ground Finches).
Certhidea
- C.olivacea (Green Warbler-finch)
- C.fusca (Grey Warbler-finch)
At ~10cm the warbler-finches are the smallest of the Darwin's finches. They have a sufficiently distinctive bill - very fine (narrow and shallow), pointed (~3x as long as deep) - that they are more likely to be confused with juvenile Yellow Warbler than with the other Darwin's finches. The two species look very similar but they are allopatric. It is easier to distinguish them from each other by knowing which island you are on than by relying on any clues gleaned from their English (or specific) names. Green is monotypic and occurs on the central and western islands - Isabella, Fernandina, Santiago, Santa Cruz and Baltra. Grey has 7 subspecies each occurring on one or two of the remaining islands - Floreana, Española, San Cristobal, Santa Fé, Genovesa, Marchena, Pinta, Darwin and Wolf.
The sexes are alike. Certhidea is considered basal within the lineage of Darwin's finches - the other genera having evolved from a Certhidea-like ancestor. |
Platyspiza
- P.crassirostris (Vegetarian Finch)
At 16cm Vegetarian Finch is one of the largest of the Darwin's finches (Large Ground Finch is a similar size). It has a short, broad, deep bill with strongly curved upper and lower edges. It tends to adopt a more upright stance than other Darwin's finches. It is uncommon but occurs on all the main islands except Española and Santa Fé, mainly in the transition zone. Males develop a black head, neck and breast when mature. It is monotypic.
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Camarhynchus
- C.pallidus (Woodpecker Finch)
- C.heliobates (Mangrove Finch)
- C.psittacula (Large Tree-finch)
- C.pauper (Medium Tree-finch)
- C.parvulus (Small Tree-finch)
In Woodpecker Finch (15cm) and Mangrove Finch (14cm) the bill is long, stout, conical and pointed (~2x as long as deep) with the upper edge curved and the lower edge straight. The plumage of Woodpecker Finch is indistinctly streaked (most other Darwin's finches are distinctly streaked). The sexes are alike. It is locally common, mainly in the highlands, on Fernandina, Isabella, Santa Cruz, San Cristobal and Santiago; it has 3 subspecies.
Mangrove Finch is critically endangered and occurs only in a small area in mangroves on the west coast of Isabella, just north of Tagus Cove. |
The 3 'tree-finches' have bills that are parrot-like - about as long as they are deep, with both the upper and lower edges curved and with the upper mandible extending as a point over the lower mandible. They all exhibit sexual dimorphism with the males developing a black head, neck and breast when mature. Large and Medium Tree-finches are the same size (13cm) and are very similar; they are larger than Small Tree-finch (11cm). Large Tree-finch is uncommon and occurs on Isabella, Fernandina, Santiago, Floreana and Santa Cruz; Medium Tree Finch is restricted to the highlands of Floreana. In Large Tree-finch the lower margin of the bill is distinctly angular and the mandibles cross at the apex. Large Tree-finch has 3 subspecies, that occurring on Isabella and Fernandina (affinis) has a slightly smaller bill. In Medium Tree-finch the lower edge of the bill is less angular and the mandibles do not cross at the apex. The bill of Small Tree-finch is relatively small and stubby. It has 2 subspecies, one of which (salvini) occurs only on San Cristobal.
Geospiza
- G.difficilis (Sharp-beaked Ground-finch)
- G.scandens (Common Cactus-finch)
- G.conirostris (Española Cactus-finch)
- G.propinqua (Genovesa Cactus-finch)
- G.magnirostris (Large Ground-finch)
- G.fortis (Medium Ground-finch)
- G.fuliginosa (Small Ground-finch)
Geospiza species all exhibit sexual dimorphism with full (super-)male plumage gradually being acquired over a sequence of 5 moults - they become entirely black apart from the undertail coverts which are white-tipped in all except Sharp-beaked where they are rufous. Females are generally brown, with paler underparts that are streaked to a variable extent. They differ in size and bill-shape and generally the larger the species the deeper is its bill.
Large Ground-finch (16cm) is the largest; its bill is deeper (at the base) than it is long and occupies the whole of the front of the head; the curve of the upper edge is 'continuous with' the forehead, though there may be a raised ridge at the junction of bill and forehead; the cutting edge is noticeably angulate (on both surfaces). It is uncommon and tends to occur singly in the arid zone. It is monotypic and can be found on most of the main islands, but not in the SE of the archipelago on Española, Floreana, San Cristobal or Santa Fé.
Large Ground-finch (16cm) is the largest; its bill is deeper (at the base) than it is long and occupies the whole of the front of the head; the curve of the upper edge is 'continuous with' the forehead, though there may be a raised ridge at the junction of bill and forehead; the cutting edge is noticeably angulate (on both surfaces). It is uncommon and tends to occur singly in the arid zone. It is monotypic and can be found on most of the main islands, but not in the SE of the archipelago on Española, Floreana, San Cristobal or Santa Fé.
Española Cactus-finch (15cm) is quite large and is similar to Large Ground-finch, except that the bill is a little more elongate. It occurs only on Española (where it is the only large-billed finch).
Genovesa Cactus-finch (14cm) is slightly smaller than Española Cactus-finch, with a slightly less deep more elongate bill and slightly larger than Common Cactus-finch with a slightly deeper, less elongate bill. It only occurs on Genovesa*, where neither of the other 2 species mentioned occur.
*With the split of "Large Cactus-finch" I would expect the 3rd subspecies (darwini), which occurs on Darwin & Wolf Islands to be placed in G.propinqua.
*With the split of "Large Cactus-finch" I would expect the 3rd subspecies (darwini), which occurs on Darwin & Wolf Islands to be placed in G.propinqua.
Common Cactus-finch (14cm) is medium-sized and has a long, pointed bill. It is usually found on Prickly Pear Cacti (Opuntia species). It is locally common. It has 4 subspecies and occurs on all the islands except Española, Fernandina and Genovesa.
Sharp-beaked Ground-finch (12.5cm) is a small species with an elongate pointed bill with fairly straight upper and lower margins. It is most similar to Small Ground-finch which has a less elongate bill with a more curved upper margin. Females are darker than in other Geospiza species. There are 3 subspecies. It occurs on Fernandina, Santiago, Genovesa, Pinta, Darwin & Wolf.
Small Ground-finch (11cm) is the smallest of the ground-finches. It has a shallower-based bill and usually shows more forehead than in Medium Ground-finch. It has a less elongate, less pointed bill than in Sharp-beaked Ground-finch. It is monotypic but quite variable. It is abundant and occurs on all the islands except Genovesa.
Medium Ground-finch (12.5cm) is a small species with a deep-based bill with a strongly-curved upper margin. It is monotypic but quite variable - this variability making it potentially confusable with both Large and Small Ground-finches. It is noticeably smaller than Large Ground-finch and usually shows a less angulate lower margin to the bill. It is only slightly larger than Small Ground-finch which has a shallower-based 'daintier' bill and usually shows a significant amount of forehead. It is abundant and occurs on all the islands except Genovesa and Española