Swallow Bird

The swallows and martins are a group of passerine birds in the family Hirundinidae which are characterised by their adaptation to aerial feeding. Swallow is used colloquially in Europe as a synonym for the Barn Swallow.
This family comprises two subfamilies: Pseudochelidoninae (the river martins of the genus Pseudochelidon) and Hirundininae (all other swallows and martins). Within the Hirundiniae, the name "martin" tends to be used for the squarer-tailed species, and the name "swallow" for the more fork-tailed species; however, there is no scientific distinction between these two groups. The family contains around 83 species in 19 genera.

The swallows have a cosmopolitan distribution across the world and breed on all the continents except Antarctica. It is believed that this family originated in Africa as hole-nesters; Africa still has the greatest diversity of species. They also occur on a number of oceanic islands. A number of European and North American species are long-distance migrants; by contrast, the West and South African swallows are non-migratory. A few species of swallow and martin are threatened with extinction by human activities, although other species have benefited from human changes to the environment and live around humans.
he swallows and martins have an evolutionary conservative body shape which is similar across the family but is unlike that of other passerines. Swallows have adapted to hunting insects on the wing by developing a slender streamlined body and long pointed wings, which allow great maneuverability and endurance, as well as frequent periods of gliding. Their body shape allows for very efficient flight, which costs 50-75% less for swallows than equivalent passerines of the same size. Swallows usually forage at around 30–40 km/h, although they are capable of reaching speeds of between 50–65 km/h when traveling.
Like the unrelated swifts and nightjars, which hunt in a similar way, they have short bills, but strong jaws and a wide gape. Their body length ranges from about 10–24 cm (3.9–9.4 in) and their weight from about 10–60 g (0.35–2.1 oz). The wings are long, pointed, and have nine primary feathers. The tail has 12 feathers and may be deeply forked, somewhat indented, or square-ended. A long tail increases maneuverability, and may also function as a sexual adornment, since the tail is frequently longer in males.In Barn Swallows the tail of the male is 18% longer than the females, and females will select mates on the basis of tail length.
The legs are short, and their feet are adapted for perching rather than walking, as the front toes are partially joined at the base. Swallows are capable of walking and even running, but they do so with a shuffling, waddling gait.The leg muscles of the river martins (Pseudochelidon) are stronger and more robust than those of other swallows.
The most common hirundine plumage is glossy dark blue or green above and plain or streaked underparts, often white or rufous. Species which burrow or live in dry or mountainous areas are often matte brown above (e.g. Sand Martin and Crag Martin). The sexes show limited or no sexual dimorphism, with longer outer tail feathers in the adult male probably being the most common distinction.
The chicks hatch naked and with closed eyes. Fledged juveniles usually appear as duller versions of the adult.