The Hirundinidae, or swallows, martins, and saw-wings, are a family of passerine birds found on all continents, including Antarctica on rare occasions. They have a unique look and are well-adapted to aerial eating. In Europe, the name “swallow” is used as a colloquial term for the barn swallow. Hirundinidae is a family of about 90 species split into 19 genera, with the most variety found in Africa, which is also where they are considered to have originated as hole-nesters. They’re also found on a few maritime islands. Long-distance migrants include a number of European and North American species; however, West and South African swallows are nonmigratory.

Pseudochelidoninae (river martins of the genus Pseudochelidon) and Hirundininae (river martins of the genus Hirundinae) are two subfamilies of this family (all other swallows, martins, and saw-wings). The term “martin” is commonly used in the Old World for square-tailed species, and “swallow” for fork-tailed species; however, this distinction does not indicate a true evolutionary split. The term “martin” is only used in the New World to refer to members of the genus Progne. (It is because of these two systems that the same species is known as sand martin in the Old World and bank swallow in the New World.)

The Hirundinidae have an evolutionarily conservative body form that is consistent throughout the group but differs from that of other passerines. Swallows have evolved a slim, streamlined body and long, pointed wings that allow for tremendous agility and endurance, as well as frequent periods of gliding, in order to hunt insects on the wing. Swallows’ body forms enable them to fly exceedingly efficiently; their metabolic rate in flight is 49–72 percent lower than that of comparable passerines of the same size.

Swallows have two foveae in each eye, allowing them to track prey with acute lateral and frontal vision. They also have relatively long eyes, which are about equivalent in length to their breadth. The long eyes allow for increased visual acuity without fighting for space within the head with the brain. Swallows have an eye morphology that is comparable to that of a raptor.

They have small bills, powerful jaws, and a wide gape, like the unrelated swifts and nightjars, who hunt in a similar manner. Their body lengths and weights vary from 10–24 cm (3.9–9.4 in) and 10–60 g (0.35–2.12 oz), respectively. The main feathers are nine and the wings are long and pointed. There are 12 feathers on the tail, which can be severely forked, slightly indented, or square-ended. A long tail improves maneuverability and may also serve as a sexual ornament, as males’ tails are often longer. The tail of a male barn swallow is 18 percent longer than that of a female, and females choose mates based on tail length.

Their legs are small, and their feet, which are partly connected at the base, are suited for perching rather than walking. Swallows can walk and even sprint, albeit with a shuffling, waddling stride. River martins’ (Pseudochelidon) leg muscles are stronger and more robust than those of other swallows.

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