Barn swallows are small birds. They range in size from 14.6 to 19.9 cm long, with a wingspan of 31.8 to 34.3 cm. They weigh between 17 and 20 g. Barn swallows are metallic blue-black above and pale beige below. They have light brown on their throat and forehead, and have a long, deeply-forked tail. Males and females are similar in appearance, though females tend to be less vibrantly colored and have shorter outer tail-streamers.
Asymmetry of physical characteristics in barn swallows tends to be transmitted to the young in distinct parent to offspring patterns. Tail asymmetry tends to pass from father to son and from mother to daughter. Alternatively, wing asymmetry does not appear to transfer at all on a reliable basis from parent to offspring.
Six subspecies of Hirundo rustica are recognized.
Range mass: 17 to 20 g.
Range length: 14.6 to 19.9 cm.
Range wingspan: 31.8 to 34.3 cm.
Other Physical Features: endothermic ; bilateral symmetry ; polymorphic
Sexual Dimorphism: male larger; male more colorful
Average basal metabolic rate: 0.3158 W.