The possible functions of calls organization in the bittern (Botaurus stellaris)

Abstract

During the reproductive period, male bitterns produce repetitive trains of signals (booms) with not yet fully understood significance for their territorial and sexual behaviour. To look for possible functions of these calls in terms of the ‘Honest Advertisement Hypothesis’ or ‘Locatability Hypothesis’, different characteristics and parameters of the vocalizations were analysed and the following evidence was obtained: (1) the number of booms per boom train, as well as the percentage of faint ‘poor’ booms, produced by 12 individuals proved to be uncorrelated with their body size or condition. (2) The highly individual variable parameters of 22 males’ vocalizations were often weakly inter‐correlated and a common organization pattern of the boom train was not recognisable. However, main element duration tended to decrease as the percentage of poor booms and the number of booms per boom train increased. (3) Boom trains of bitterns mixing full and poor booms, were characterised by main elements of variable duration uttered at regular intervals, while, for the other individuals, main elements of similar duration were uttered at longer intervals in the final part of the boom train. Furthermore on the basis of morphological modifications of the neck evaluated in 12 individuals, some considerations on the mechanism of sound production were made. The boom trains organizations did not fully match the expectations of the Honest Advertisement Hypothesis and of the Locatability Hypothesis. Bitterns appeared, in fact, to look for a trade off between boom train length and main element duration: longer boom trains made up mainly of poor booms may be due to the need to prolong the trains, reducing the duration of the main elements (which would then be poor) or to that of prolonging trains formed by shorter casually produced poor booms.

Key words:

• Bittern
• Botaurus stellaris
• Vocalizations
• Honest signal
• Locatability