Tersina is a genus of tanager. It contains a single species, the swallow tanager (Tersina viridis). It is found widely throughout South America, from eastern Panama to far northern Argentina.
It is 14 to 15 cm length. It has a very strong sexual dimorphism. The plumage of the male is bright blue with a black mask and throat; It has a white belly and chrysum. The female is bright green on the back and greenish yellowish on the undersides, with striated flanks.
The species T. viridis was first described by the German zoologist Johann Karl Wilhelm Illiger in 1811 under the scientific name Hirundo viridis; its type locality is: «Sandwich Islands, error, amended for eastern Brazil.»
The genus Tersina was proposed by the French naturalist Louis Jean Pierre Vieillot in 1819, with Tersina coerulea as the type species, which turned out to be a later synonym for the present species.
The generic name Tersina derives from the French word «tersine», used by Buffon to designate an unidentified bird; and the name of the species «viridis» from Latin which means: green, in reference to the color of the female.
Recent extensive phylogenetic studies demonstrate that the present species is a close relative of Cyanerpes, in a Dacninae subfamily.
According to theclassifications of the International Ornithological Congress (IOC)  and Clements Checklist / eBird v.2019 , three subspecies are recognized, with their corresponding geographic distribution:
- Tersina viridis occidentalis (P.L. Sclater), 1855 – east of Panama to Venezuela, the Guianas, north of Bolivia and north of Brazil.
- Tersina viridis griscens Griscom, 1929 – Sierra Nevada de Santa Marta, northeast Colombia.
- Tersina viridis viridis (Illiger), 1811 – eastern Bolivia to Paraguay, eastern Brazil, and northeastern Argentina.
Distribution and habitat
It is widely distributed in South America in much of Colombia, north and south of Venezuela, south of Guyana, Suriname and French Guiana, extreme north and northwest of Brazil, much of Brazil south of the Amazon River (except the interior of the northeast, parts of central east and south), west and east of Ecuador. east of Peru, north and east of Bolivia, north and east of Paraguay, to the extreme northeast of Argentina (Misiones); also in the extreme east of Panama and in Trinidad.
This species is widely disseminated and visible, generally in semi-open terrain, on the edges of humid forests, in clearings with scattered trees and in gallery forests, up to 1800 m of altitude. It is nomadic and can be seasonally or locally common.
It feeds on fruits and insects, which it can catch in flight. Due to the shape of the beak and head, it is capable of collecting various fruits, taking them and storing them in a hidden place.  Consume a variety of fruits, including berries and avocado, as well as various insects, such as termites, grasshoppers, and ants. 
The female digs a ravine and makes the nest at the bottom of the tunnel, where she lays two to four eggs,  which she incubates for 13 to 17 days. The chicks, fed only by the female, leave the nest after about 24 days. The male is only in charge of guarding the nest.
The call is a sharp and non-musical «tziip», distinctive and often given in flight.
- BirdLife International (2020). «Tersina viridis». IUCN Red List of Threatened Species 2020.3. ISSN 2307-8235. Retrieved April 5, 2021.
- Vieillot, L.J.P. (1819). Nouveau Dictionnaire d’Histoire naturelle, appliquée aux arts, à l’agriculture, à l’économie rurale et domestique, à la médecine, etc. (in French). Take 33. 568 pp. + 2 tt. Paris: Déterville. Tersina and Tersina coerulea, original citation p.401. Available at Biodiversitas Heritage Library.
- Illiger, J.K.W. (1811). Prodromus systematis mammalium et avium additis terminis zoographicis utriusque classis, eorumque versione germanica (in Latin). xviii + 302 pp. Berolini (Berlin): C. Salfield. Hirundo viridis, original citation, p.229. Available at Biodiversitas Heritage Library. doi: 10.5962 / bhl.title.106965.
- Procnias ventralis Richmond Index – Species and Subspecies – Division of Birds at the National Museum of Natural History, Washington, D.C.