Shelduck calls are sexually dimorphic. Calls of males are high-pitched and whistling, whereas female calls sound deep and croaking. Males have different call types; in females 7 call types are known. According to preliminary observations, both males and females react specifically to calls of their partners. Individual call differences can be heard also by man. This paper aims at finding parameters of individuality, as revealed by sonograms, in the male trill-call. 1. Candidate cues for individual recognition should be (a) variable between individuals and (b) constant within individuals. 'lndividuality Index' we call the relation between average individual variation coefficient and total variation coefficient of one character in the population under study. 2. For individual recognition, the receiver could make use of several parameters. Sheldrake trill calls are individually different. Different parameters are the form of the first element and presence/absence of secondary elements. Duration and highest frequency of the calls have not been demonstrated to be reliable individual cues.
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