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.

Barn swallows are semi-colonial, settling in groups from a single pair to a few dozen pairs, particularly in larger wooden structures housing animals. The same individuals often breed at the same site year after year, although settlement choices have been experimentally shown to be predicted by nest availability rather than any characteristics of available mates. Because it takes around 2 weeks for a pair to build a nest from mud, hair, and other materials, old nests are highly prized.

This species breeds across the Northern Hemisphere from sea level to 2,700 m (8,900 ft), but to 3,000 m (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 the house martin replaces it in Europe in urban areas. However, in Honshū, Japan, 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 habitat choice, 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 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 synchronized 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, the Falkland Islands, and even Antarctica.

https://en.wikipedia.org/wiki/Barn_swallow