The Barn Swallow (Hirundo rustica) is the most widespread species of swallow in the world. A distinctive passerine bird with blue upperparts, a long, deeply forked tail and curved, pointed wings. It is found in Europe, Asia, Africa and the Americas. In Anglophone Europe it is just called the Swallow; in Northern Europe it is the only common species called a "swallow" rather than a "martin".

There are six subspecies of Barn Swallow, which breed across the Northern Hemisphere. Four are strongly migratory, and their wintering grounds cover much of the Southern Hemisphere as far south as central Argentina, the Cape Province of South Africa, and northern Australia. Its huge range means that the Barn Swallow is not endangered, although there may be local populatin declines due to specific threats, such as the construction of an international airport near Durban.

The Barn Swallow is a bird of open country which normally uses man-made structures to breed and consequently has spread with human expansion. It builds a cup nest from mud pellets in barns or similar structures and feeds on insects caught in flight. This species lives in close association with humans, and its insect-eating habits mean that it is tolerated by man; this acceptance was reinforced in the past by superstitions regarding the bird and its nest. There are frequent cultural references to the Barn Swallow in literary and religious works due to both its living in close proximity to humans and its conspicuous annual migration. The Barn Swallow is the national bird of Estonia.

The adult male Barn Swallow of the nominate subspecies H. r. rustica is 17–19 cm (6.7–7.5 in) long including 2–7 cm (0.8–2.8 in) of elongated outer tail feathers. It has a wingspan of 32–34.5 cm (12.6–13.6 in) and weighs 16–22 g (0.56–0.78 oz). It has steel blue upperparts and a rufous forehead, chin and throat, which are separated from the off-white underparts by a broad dark blue breast band. The outer tail feathers are elongated, giving the distinctive deeply forked "swallow tail." There is a line of white spots across the outer end of the upper tail.

The female is similar in appearance to the male, but the tail streamers are shorter, the blue of the upperparts and breast band is less glossy, and the underparts more pale. The juvenile is browner and has a paler rufous face and whiter underparts. It also lacks the long tail streamers of the adult.

The song of the Barn Swallow is a cheerful warble, often ending with su-seer with the second note higher than the first but falling in pitch. Calls includewitt or witt-witt and a loud splee-plink when excited. The alarm calls include a sharp siflitt for predators like cats and a flitt-flitt for birds of prey like the Hobby. This species is fairly quiet on the wintering grounds.

The distinctive combination of a red face and blue breast band render the adult Barn Swallow easy to distinguish from the African Hirundo species and from the Welcome Swallow (Hirundo neoxena) with which its range overlaps in Australasia. In Africa the short tail streamers of the juvenile Barn Swallow invite confusion with juvenile Red-chested Swallow (Hirundo lucida), but the latter has a narrower breast band and more white in the tail.

The preferred habitat of the Barn Swallow is open country with low vegetation, such as pasture, meadows and farmland, preferably with nearby water. This swallow avoids heavily wooded or precipitous areas and densely built-up locations. The presence of accessible open structures such as barns, stables, or culverts to provide nesting sites, and exposed locations such as wires, roof ridges or bare branches for perching, are also important in the bird's selection of its breeding range.

It breeds in the Northern Hemisphere from sea level to typically 2,700 metres (8,900 ft), but to 3,000 metres (9,800 ft) in the Caucasus and North America, and it is absent only from deserts and the cold northernmost parts of the continents. Over much of its range, it avoids towns, and in Europe is replaced in urban areas by the House Martin. However, in Honshū, the Barn Swallow is a more urban bird, with the Red-rumped Swallow (Cecropis daurica) replacing it as the rural species.

In winter, the Barn Swallow is cosmopolitan in its choice of habitat, avoiding only dense forests and deserts. It is most common in open, low vegetation habitats, such as savanna and ranch land, and in Venezuela, South Africa and Trinidad and Tobago it is described as being particularly attracted to burnt or harvested sugarcane fields and the waste from the cane. In the absence of suitable roost sites, they may sometimes roost on wires where they are more exposed to predators. Individual birds tend to return to the same wintering locality each year and congregate from a large area to roost in reed beds. These roosts can be extremely large, one in Nigeria had an estimated 1.5 million birds. These roosts are thought to be a protection from predators, and the arrival of roosting birds is synchronised in order to overwhelm predators like African Hobbies. The Barn Swallow has been recorded as breeding in the more temperate parts of its winter range, such as the mountains of Thailand and in central Argentina.

Migration of Barn Swallows between Britain and South Africa was first established on 23 December 1912 when a bird that had been ringed by James Masefield at a nest in Staffordshire, was found in Natal. As would be expected for a long-distance migrant, this bird has occurred as a vagrant to such distant areas as Hawaii, Bermuda, Greenland, Tristan da Cunha and the Falkland Islands.

The Barn Swallow is similar in its habits to other aerial insectivores, including other swallow species and the unrelated swifts. It is not a particularly fast flier, with a speed estimated at about 11 m/s, up to 20 m/s and a wing beat rate of approximately 5, up to 7–9 times each second, but it has the manoeuvrability necessary to feed on flying insects while airborne. It is often seen flying relatively low in open or semi-open areas.

The Barn Swallow typically feeds 7–8 metres (23–26 ft) above shallow water or the ground, often following animals, humans or farm machinery to catch disturbed insects, but it will occasionally pick prey items from the water surface, walls and plants. In the breeding areas, large flies make up around 70% of the diet, with aphids also a significant component. However, in Europe, the Barn Swallow consumes fewer aphids than the House or Sand Martins. On the wintering grounds, Hymenoptera, especially flying ants, are important food items. When egg-laying, Barn Swallows hunt in pairs, but will form often large flocks otherwise.

Isotope studies have shown that wintering populations may utilise different feeding habitats, with British breeders feeding mostly over grassland, whereas Swiss birds utilised woodland more.Another study showed that a single population breeding in Denmark actually wintered in two separate and different areas.

The Barn Swallow drinks by skimming low over lakes or rivers and scooping up water with its open mouth. This bird bathes in a similar fashion, dipping into the water for an instant while in flight.

Swallows gather in communal roosts after breeding, sometimes thousands strong. Reed beds are regularly favoured, with the birds swirling en masse before swooping low over the reeds. Reed beds are an important source of food prior to and whilst on migration; although the Barn Swallow is a diurnal migrant which can feed on the wing whilst it travels low over ground or water, the reed beds enable fat deposits to be established or replenished.

The male Barn Swallow returns to the breeding grounds before the females and selects a nest site, which is then advertised to females with a circling flight and song. The breeding success of the male is related to the length of the tail streamers, with longer streamers being more attractive to the female. Males with longer tail feathers are generally longer-lived and more disease resistant, females thus gaining an indirect fitness benefit from this form of selection, since longer tail feathers indicate a genetically stronger individual which will produce offspring with enhanced vitality.Males in northern Europe have longer tails than those further south; whereas in Spain the male's tail streamers are only 5% longer than the female's, in Finland the difference is 20%. In Denmark, the average male tail length increased by 9% between 1984 and 2004, but it is possible that climatic changes may lead in the future to shorter tails if summers become hot and dry.

Males with long streamers also have larger white tail spots, and since feather-eating bird lice prefer white feathers, large white tail spots without parasite damage again demonstrate breeding quality; there is a positive association between spot size and the number of offspring produced each season.

Both sexes defend the nest, but the male is particularly aggressive and territorial. Once established, pairs stay together to breed for life, but extra-paircopulation is common, making this species genetically polygamous, despite being socially monogamous. Males guard females actively to avoid being cuckolded. Males may use deceptive alarm calls to disrupt extrapair copulation attempts toward their mates.

As its name implies, the Barn Swallow typically nests inside accessible buildings such as barns and stables, or under bridges and wharves. The neat cup-shaped nest is placed on a beam or against a suitable vertical projection. It is constructed by both sexes, although more often by the female, with mud pellets collected in their beaks and lined with grasses, feathers, algae or other soft materials. Barn Swallows may nest colonially where sufficient high-quality nest sites are available, and within a colony, each pair defends a territory around the nest which, for the European subspecies, is four to eight square metres (45 to 90 square feet) in size. Colony size tends to be larger in North America.