Smaller than white-throated swallow, this species differs in lacking a black breast band, and having a bright chestnut cap and a black streak across the vent. It is very fast and agile in flight. The very thin tail streamers are often difficult to see.
Juvenile has the chestnut of adult replaced largely by brown and is less glossy blue above.
Usually found near water, often breeding under bridges.
A sharp metallic 'tchik'.