Vegetarian finch

Systematics

The vegetarian finch is one of Darwin's finches, a group of closely related birds which evolved on the Galápagos Islands. The group is related to the Tiaris grassquits, which are found in South America and the Caribbean. An ancestral relative of those grassquits arrived on the Galápagos Islands some 2–3 million years ago, and the vegetarian finch is an early evolutionary radiation from that ancestor.

When Darwin first collected the species in 1835, he assumed it was a finch. John Gould, who formally described the vegetarian finch in 1837, agreed and assigned it to the genus Fringilla. By 1841, Gould had changed his mind, and moved the species to the genus Camarhynchus, lumping it with the ground and cactus finches. Robert Ridgway separated it from the other species in 1896, assigning it to a new genus Platyspiza. DNA research has now shown that all Darwin's "finches" are actually tanagers.

The vegetarian finch is the sole member of the genus Platyspiza, which some taxonomists still subsume into the genus Camarhynchus. The genus name Platyspiza comes from the Greek platus, meaning "broad" and spiza, meaning "finch". The specific name crassirostris comes from the Latin crassus, meaning "heavy" or "thick" and rostris, meaning "-billed" (rostrum = bill). The "vegetarian" of its common name refers to its primary diet.

Description

The vegetarian finch is one of the largest Galápagos finches, measuring 16 cm (6.3 in) in length and ranging from 29 to 40 g (1.0 to 1.4 oz) in mass. Its upright stance is described as "parrot-like". Its beak is broad and stout, with a strongly curved culmen. The male's upperparts are olive-colored while his underparts are whitish, with smudgy streaking on the lower breast and flanks; some birds show rufous on the underparts. His lower flanks and undertail coverts are buffy. He has a black hood, throat, breast and upper flanks. His iris is dark, and his bill is black in the breeding season and horn-colored during the rest of the year. The female is principally brown above and off-white below, with a buffy rump and flanks. She is streaked with brown on the face, crown, upperparts, throat, breast and flanks, and shows two indistinct buffy wingbars on her brown wings. Her beak is two-toned; the upper mandible ranges in color from dusky brown to black, while the lower mandible is dull orange or dull pink. The immature male is intermediate between the adult male and the adult female. While he shows blackish on his face and throat, he is more streaked below than is the adult male.

Voice

The song of the vegetarian finch is nasal and drawn out, with each note lasting about two seconds. Transcribed as ph'wheeeuuuuu-íííúúú, it is accented towards the end. The bird's primary call is high-pitched and squealing, said to resemble the sound of a radio tuner. It also gives a whiny pheep.

Range and habitat

Endemic to the Galápagos, the vegetarian finch is found on eight islands: San Cristóbal, Santa Cruz, Floreana, Isabela, Marchena, Santiago, Pinta and Fernandina. Although it was previously found on Pinzón and Santa Fé, the species is now extinct on both islands. It is found from 0 to 500 m (0 to 1,640 ft) above sea level. Although it is most common in montane evergreen forest, particularly the transition zone, its range also extends up into the humid zone and down into the arid zone.

Breeding

Little is known about the breeding ecology of this species. It breeds primarily in the wet season, building a grassy domed nest with a side entrance. Courtship feeding is known to occur throughout courtship and incubation, with some pairs passing food items back and forth several times.

Feeding

As its name suggests, the vegetarian finch is largely a plant-eater. It feeds primarily on buds, leaves, flowers and fruit, and will strip the bark off twigs to get to the cambium and phloem which lies underneath. Although it forages mainly in trees, it will descend to the ground to search for fallen fruits and young plant shoots. It also occasionally eats caterpillars. It feeds primarily at mid-levels, in what has been described as a "rather leisurely" manner. Because its principal food items are soft, the vegetarian finch has a beak morphology unlike those of Darwin's finches which specialize on hard seeds. Described as "parrot-like", the beak is small and stout, with a steep profile and a strong curve in the upper mandible. Its primary function is food manipulation at the tip, rather than seed crushing at the base. The vegetarian finch has a disproportionately large gizzard, as well as a long intestine and a disproportionately small heart. These allow it to process the "relatively indigestible" leaves and buds that make up a large proportion of its diet.

Conservation and threats

Although the vegetarian finch is uncommon, it is widespread across the Galápagos Islands. Its numbers appear to be stable, though they have not been quantified. The International Union for the Conservation of Nature lists it as a species of Least Concern, as neither its population size nor its range size approach thresholds for concern. However, like all endemic wildlife on the Galápagos Islands, it is impacted by some human activities. Fires, overgrazing by domestic and feral animals, and the introduction of exotic species are among the most serious threats it faces. It is found in seven of the Important Bird Areas established on the islands.