Song: A long, chattering, babbling series of twittering phrases ending in a dry rattle — phonetically roughly "vit-vit-vit-vitrr...tvitvitvit"; it likes to sing while perched on a telephone wire or a roof ridge.
Call: A distinct "vit" or "tvitt" — heard in flight and at the nest; variations in frequency are used for communication within the flock.
Alarm: A sharp "sjirrr" when a bird of prey appears; it triggers an immediate reaction throughout the colony, with collective mobbing of the intruder.
Listen to the Barn Swallow on xeno-canto and see observations on Artportalen (the Swedish species observation system).