[3] Its common name and specific epithet (which means "large-tailed") both refer to the long, deeply forked, somewhat swallow-like tail.

The swallow-tailed hummingbird was formally described in 1788 by the German naturalist Johann Friedrich Gmelin in his revised and expanded edition of Carl Linnaeus's Systema Naturae.

[5][6] In 1929 Carl Eduard Hellmayr argued that Gmelin conflated two different species and that his diagnosis applied to Brisson's specimen from Cayenne; Sloane's Jamaican bird was probably the red-billed streamertail.

[10][12] With a total length of 15–17 cm (6–6+1⁄2 in), nearly half of which is made up by the tail, and weighing up to 9 g (0.32 oz), this is a relatively large hummingbird.

Its wings are also nearly 8 cm long – quite much for its size by hummingbird standards – though its bill is only of mediocre length, with c. 21 mm (0.83 in) not longer in absolute terms than that of many smaller relatives.

The swallow-tailed hummingbird is virtually unmistakable, although occasionally confused with the male violet-capped woodnymph (Thalurania glaucopis).

[13] It generally avoids the rainforest found throughout most of the Amazon Basin, and only extends locally into this region along the southern and eastern edge, in the relatively open habitats along the lowermost sections of the Amazon River, including Marajó Island, and upstream to around the Tapajós River, and in isolated enclaves of woodland or savanna-like habitats within the Amazon (including so-called "Amazonian Caatinga") in south-eastern Peru (upper Urubamba River and Pampas del Heath),[14] southern Suriname (Sipaliwini Savanna),[15] central Brazil, and northern Bolivia.

It occurs in virtually any semi-open habitat; even gardens and parks within major cities[16] such as Rio de Janeiro and São Paulo.

It avoids the interior of humid forest, but does occur in openings or along the edge; the swallow-tailed hummingbird is most common among savanna-like vegetation.

[18] It is aggressive and will defend rich food sources from other nectarivores;[3] due to its size, it is generally dominant over other species of hummingbirds.

Smaller mammals, such as the common marmoset (Callithrix jacchus) may occasionally plunder swallow-tailed hummingbird nests, despite the birds' attempts to defend their offspring.

[18] The swallow-tailed hummingbird mainly forages at mid-levels, but good food sources are exploited from anywhere near ground level right up to the tree tops.

It chiefly feeds on flower nectar, particularly from Fabaceae,[20] Gesneriaceae, Malvaceae (especially Bombacoideae and Malvoideae), Myrtaceae, Rubiaceae and epiphytic Bromeliaceae.

The nest is a cup-shaped structure lined with soft plant fibres and clad on the outside with lichen and mosses, held together with spider webs.