Shifts in sound features of the duetting pallid swifts apus pallidus L.

Bioacoustic characteristics of the screaming calls of the pallid swift (Apus pallidus) emitted by the breedings pairs in antiphonal sequence were studied. The calls alternate with precise time intervals. In 44% of our records each individual maintains constant spectrographic characteristics, while in 56% a mate shifts its sound features on to the partner ones. It is suggested that the last part of the call maintains the highest personal features allowing vocal individual recognition, and that the calls have a territorial function against conspecific intrusion in the nest-holes of the breeding pair.