GALAPAGOS Finches

The Galápagos Finches (or Darwin’s Finches as they are known since the mid of the 20^th century) were up to the 1970s placed in the Geospizinae subfamily in Fringillidae: (CARDINALS, BUNTINGS and FINCHES), then in Emberizinae subfamily (New World sparrows and Old World buntings) in Emberizidae: BUNTINGS, CARDINALS AND TANAGERS (sometimes in Emberizinae in Fringillidae), and after DNA investigation
(with the Emberizidae reduced to OLD WORLD BUNTINGS), in the 2010s into
Coerebinae (dome-nesting Tanagers) subfamily within Thraupidae: TANAGERS.

So, from finches, they became buntings, and only recently, tanagers. However, they kept being called Finches (also in Spanish on the ‘archipiélago de Colón’).

Darwin himself thought these Finches to be a mix of wrens, blackbirds, finches and "gross-bills", but John Gould told him they all were actually ‘a series of ground Finches which are so peculiar [as to form] an entirely new group, containing 12 species’.

Ornithologist James Bond (whose name was appropriated by writer Ian Fleming for his fictional British spy of the same name) in 1950 already pointed out that the Lesser Antillean genus Melanospiza (now in Coerebinae as well) bears a remarkable resemblance to Darwin’s Finches’ Geospiza.

Earlier, Coerebinae (Honeycreepers) were part of (Peters) Emberizidae (Buntings, Tanagers, Honeycreepers) with Bananaquit and Diglossa Flowerpiercers (or Flowerpeckers, not to be confused with the Oriental and Australian Dicaeidae family of Flowerpeckers), or (Wetmore) Parulidae (New World Warblers) with Bananaquits (and sometimes Conebills), or as family Coerebidae (Neotropical honeycreepers), ultimately monotypic with only the Bananaquit left.
Neotropical honeycreepers are now listed in Hemithaupinae and Dacninae within Thraupidae.

At the other hand, the Neotropical Euphonia and the Chlorophonia were formerly placed in the tanager family Thraupidae due to their similar appearance and are now placed in the Euphoniinae subfamily within the Fringillidae: FINCHES, EUPHONIAS AND HAWAIIAN HONEYCREEPERS.

The Galápagos Finches are thought to have evolved from 1 mainland species (or possibly a Cocos Island species that reached the islands long ago). The island finches belong to a distinct subfamily (Coerebinae) of tanagers (Thraupidae), endemic to the Galápagos (Ecuador) and Cocos Island (Costa Rica). Cocos Island is several hundred km north of the Galápagos and about the same distance from the mainland. It supports only 1 finch species, the Cocos Island finch Pinaroloxias inornata. This finch species, unlike those on the Galápagos, has had no opportunity for further geographic isolation (other than from the mainland). Thus, its gene pool has never been split and additional species have never evolved there. The Cocos finch is a generalist feeder, in contrast to many of those on the Galápagos which exhibit specialist feeding niches. Today, three to ten species occur side by side on some of the volcanic islands.

A 2001 study identifies the grassquit genus Tiaris, and specifically the species Tiaris obscura (Dull-coloured grassquit), earlier known as Sporophila obscura (which genus is now in Sporophilinae), as the nearest living relative of Darwin's Finches among the species surveyed. Darwin's Finches diverged from the Tiaris group shortly after the various extant species of Tiaris diverged from one another. The initial adaptive radiation of the Tiaris group apparently occurred on the Caribbean islands and then spread to Central and South America, from where the ancestors of Darwin's Finches departed for the Galápagos Islands approximately 2.3 million years ago, at the time of the dramatic climatic changes associated with the closure of the Panamanian isthmus and the onset of Pleistocene glaciation. (Tiaris was since the 1970s grouped together with the Darwin's Finches, among others, in Emberizinae within Emberizidae)

The difference in mitochondrial DNA between Darwin's Finches and the Tiaris species suggests the ancestors arrived on the Galapagos two to three million years ago. Somehow they managed to cross open ocean for 1,000 km (600mls) and successfully establish a breeding colony. There has been some debate in the past on whether Cocos Island could have been the jump-off point for the finches to reach Galapagos. Here, a solitary species of finch resides today, but molecular genetic data has eliminated that possibility; the data demonstrated a phylogenetic origin of the Cocos finch after an initial evolutionary split among Darwin's Finches on the Galapagos (Petren et al. 1999, Sato et al. 1999). This evidence indicates that ancestral finches first colonized the Galapagos, after which the populations began to diverge, only later producing what is known today as the Cocos finch. Two scenarios are plausible: the Cocos finch (Pinaroloxias) could potentially have arisen from an ancestral warbler finch (Certhidea fusca) that colonized Cocos Island; or the Cocos finch may have appeared on the Galapagos Islands, with the species colonizing Cocos island later, eventually becoming extinct on the Galapagos. The black plumage and song of the Cocos finch so closely resembles that of the Galapagos Islands Finches, that the second possibility is greatly favored by many.

Taxonomic controversy

By 2016 it was found that Tiaris is polyphyletic (a polyphyletic taxon is defined as one that does not include the common ancestor of all members of the taxon), and the type species olivaceus is sister to a group of largely Caribbean and Galapagos genera. Two of the five species, Tiaris obscurus (the word Tiaris is masculine) and Tiaris fuliginosus are sisters, form a strongly supported monophyletic group and are geographic replacements of each other, but together they are sister to the Galapagos Finches, and thus a new genus must be named for them unless they and all genera of Galapagos Finches (plus Loxipasser and Loxigilla) are merged into a single genus; the latter radical treatment would, however, be consistent with the estimated age of the group, i.e. only 2 MY. It was decided for recognition of Asemospiza for obscurus and fuliginosus.

One of the 29 birds involved, the nectarivore Bananaquit (Coereba flaveola), with its curved bill, is in a basal position of the phylogeny of this group, and a real ‘super-tramp’ with 40 subspecies**, listed continuously for over a half century, next to the Jamaican type species, widespread of over much of South and Central America and on the West Indian islands (where it originated) with significant plumage differentiation and genetic divergence of local populations. According to gene-flow models, however, there is still sufficient genetic connectivity that all populations be considered to be of the same species, after in February 2010, the International Ornithological Congress listed bahamensis and bartholemica as proposed splits from C. flaveola.

Early in this century, a fourth Darwin’s Finch genus was created, extracting the genus Platyspiza from Camarhynchus for the Vegetarian Finch, that mainly eats fruits. Other new genera that were created in Coerebinae recently are 3x Melopyrrha out of Loxigilla, and out of Tiaris 1x Phonipara, 2x Asemospiza, as described above as well an additional species to Melanospiza.

However, current insights in bird taxonomy are of that state that 28 of the 29 species in the group of Coerebinae are considered to be lumped in one genus, Tiaris (with the granivore olivaceus the type species), as they are closely related, despite the variation in plumage and bill shapes. (Fjeldså p. 317).

This is in contrast to the polymorphism among the 15 Darwin’s Finches. However, the species have continued to interbreed or hybridise, after diversifying when they first arrived on the islands. Including subspecies, there are 33 birds.

Lumping all 28 under Tiaris, would mean creating another specific name for Certhidea olivacea to prevent a mix with the type species Tiaris olivaceus.

Diet

Five of the Galápagos Finches mainly eat insects, three seeds, one fruits, the others a combination of nuts and fruits, or insects and fruits.

As only few flowering species in the Galapagos produce copious nectar, there are no true nectarivore birds here, but the Cactus Finches pollinate the yellow flowers of the prickly pear (cactus), of which six endemic species occur in the genus Opuntia, and which are a prime source of food for the local giant tortoises.

** the Australasian Golden Whistler, Pachycephala pectoralis once had 72, reduced to 67 in the 1970s, when the New Guinean/Northern Australian birds were upgraded to full species, but split into 6 more species in 2003, when this happened to Wallacean and Pacific birds; Turdus poliocephalus, Island Thrush is the current world record holder with 50 subspecies not counting itself, spread over SE Asia and the Pacific.

Source: The Howard and Moore complete checklist of the birds of the world, Wikipedia
Fjeldså J, Christidis L & Ericson PGP (Eds) (2020) The Largest Avian Radiation. The Evolution of Perching Birds or the Order Passeriformes Barcelona, Lynx Edicions

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13 October, 2022