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= Green-headed hillstar = The green-headed hillstar (Oreotrochilus stolzmanni) is a species of hummingbird (Trochilidae) native to the high altitude, mountainous grasslands between Southern Ecuador and Northern Peru. It also frequents alpine forests, which are native to the Polylepis and Puya plant species. These habitats are located between 3,600 and 4,200m above sea level.

The green-headed hillstar received its name due to its bronze and green head and gorget of bright emerald green plumage. The species was first discovered by English ornithologist Osbert Salvin in 1895 and named after Polish ornithologist Jean Stanislaus Stolzmann.

The green-headed hillstar is considered a relatively common bird in the alpine regions of Ecuador and Peru and its conservation status is currently 'Least Concern'.

Taxonomy
Hummingbirds (Trochilidae) such as the green-headed hillstar are considered members of the Apodiformes bird order, along with swifts (Apodidae) and treeswifts (Hemiprocnidae). Within the hummingbird family, Hillstars (Oreotrochilus) are a genus of hummingbirds that inhabit the alpine and temperate Andes in South America between the altitudes of 1,200 and 5,200m (3,900-17,100 ft). The hillstar genus also includes the Andean hillstar (Oreotrochilus estella), Ecuadorian hillstar (Oreotrochilus chimborazo), white-sided hillstar (Oreotrochilus leucopleurus), black-breasted hillstar, (Oreotrochilus melanogaster), wedge-tailed hillstar (Oreotrochilus adela) and a new species discovered in 2017; the blue-throated hillstar (Oreotrochilus cyanolaemus).

Evolution
The green-headed hillstar was originally considered a sub-species of the Andean hillstar until it was officially elevated to species status by the Handbook of the Birds of the World and BirdLife Taxonomic Checklist version 4 in December of 2019 and the International Ornithological Committee’s World Bird Names version 10.2 in 2020.

The two species were split as a result of field studies finding significant differences in plumage and the green-headed hillstar having a stronger mitochondrial DNA relationship to the black-breasted hillstar.

Description
The green-headed hillstar has a medium-sized slightly down-curved bill and a small rounded head which rests on a plump body. It is approximately 13-14cm (5.1-5.5 in) in size and weighs between 7.9-8.4g (0.27-0.29 oz).

The species is sexually dimorphic. Adult males have bronzed green plumage on the crown, a metallic emerald green gorget, a black band between gorget and a white chest, a white underbelly with a vertical line of black feathers in the centre, dull bronze central rectrices a whitish tail. Adult females have the same bronzed green crown, a pale white gorget with grey spots, a white chest and underbelly, white webbing on the outer rectrices and a black and green tail. Juveniles of the species appear similar to adult females.

Distribution and Habitat
The green-headed hillstar is native to Peru and Ecuador, specifically between extreme Southern Ecuador and central Peru. It is not a migratory hummingbird species, rather it is sedentary and only roams seasonally to lower altitudes.

Its primary habitat is the alpine plains and forests where there are an abundance of flowers as a food source. However, green-headed hillstars have also been known to seek shelter in the eaves of buildings in villages high up in the Andes. The species is often spotted either perched on rocks or Puya clusters or occasionally on the ground in clearings. Females are more commonly found in gorges and valleys with more densely packed flora as these are some of the best nesting areas for high rates of clutch survival. Males also inhabit these gorges and valleys, but frequently occupy the rocky and barren areas as well.

Behaviour
The green-headed hillstar is usually sighted alone or in pairs, except during night-time, when groups gather in crevices and caves along the mountainside to roost. It is thought that this roosting behaviour exists to combat very cold nights at such a high altitude and as a way to evade nocturnal predators.

When foraging for food, the green-headed hillstar only searches within three metres of the ground. In contrast to the majority of hummingbird species, the green-headed hillstar perches on the stem of a flower to feed, instead of hovering. This is an energy conservation method as hovering for long periods in the thin air at high altitudes drains significant energy.

Males are the more territorial sex when claiming feeding areas, however both sexes are territorial in nature, which is typical of the Oreotrochilus genus.

Diet:
The species’ diet consists of majority pollen and nectars of the native Puya and Polypelis plants, as well as Chuquiraga, Cajophora and cacti. It will occasionally engage in a practice called ‘hawking’ to catch flying insects.

Breeding:
Breeding season occurs primarily between February and June but can also stretch to August. Adult females are ready to have their first clutch of eggs by two years old. The females build soft nests shaped like cups out of moss, plant materials and occasionally strands of wool or fur. The nests are constructed in rock cavities, under overhanging rock, under the eaves of buildings or opportunistically in open barns. Nests are almost always constructed near a reliable food source. Clutches normally include two eggs and the incubation period is 19-21 days, which the female undergoes alone. When hatched, chicks are dark in colour, with two layers of down on their back and remain fledglings for 36-40 days.

Conservation Status
The green-headed hillstar is classified on the International Union for Conservation of Nature’s Red List of Threatened Species as a species of ‘Least Concern’. This is attributed to their habitats not being suitable for agriculture, deforestation or man-made infrastructure.