After a short description of the repertoire of the species (including the purr-call, the chatter-call and the distress-call of both males and females), the paper focuses on the behavioural significance and functions of the vocalizations. These were deduced from play-back experiments (some data comes from the literature). The purr-call appears to be principally a sexual signal (given only by males), acting as a pre-mating isolating mechanism; the chatter-call, given by both sexes, acts mostly as a territorial signal (including also individual recognition, an aspect which is not detailed in the present study). Particular attention is given to variations between and within populations (Mediterranean and Atlantic colonies). There are both micro- and macro-geographic variations (on the purr-call only). Play-back tests show that birds do recognize calls from their own geographic origin. It is suggested that there is a biological function of the macro-geographic variation in calls (although morphometric studies did not reveal this fact) and that it may have consequences on species-specific recognition. The potential importance of such a mechanism is stressed for speciation process.