By Konrad Ansorge
Abstract The typical screaming calls of the Common Swift Apus apus include a scream followed by a trill. The pitch and the duration of the different parts of the calls produced by breeding birds in nestboxes vary within and between individuals; these differences do not signify the bird’s sex and probably depend largely on the degree of agitation of the individual. However, the trill part of the screaming call is a reliable indicator of a bird’s sex. Female Common Swifts show an average time lapse between the trills of ≥25 ms whereas that of males is ≤20 ms. The fundamental frequency at the end of the screaming call of females is modulated by ≤40 Hz, whereas that of males is modulated by ≥50Hz.
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