Source–filter theory assumes that calls are generated by a vocal source and are subsequently filtered by the vocal tract. The air in the vocal tract vibrates preferentially at certain resonant frequencies, called formants. Formant frequencies can be a good indicator of the caller's characteristics, such as sex, age, body size or individual identity. Although source–filter theory was originally proposed for mammals, formants are also observed in birds, and some bird species have been shown to perceive formants. In this study, we evaluated the hypotheses that formant frequencies (1) are an indicator of body size and (2) can be used for individual discrimination by a nocturnal bird species, the corncrake (Crex crex). We analysed calls of 104 males from Poland and the Czech Republic. Linear regression models showed that the males with a longer head (including the bill length) had a significantly lower formant dispersion and lower fourth and fifth formant frequencies. However, we found no significant relationships between body weight and any filter-related acoustic measurement. The formant frequencies had smaller within- than between-individual coefficients of variation. This characteristic of the formant frequencies implies a high potential for individual coding. A discriminant function analysis correctly assigned 94.8% of the calls to the caller based on formants from second to fifth. Our results indicated that the formant frequencies are a weak indicator of the body size of the sender in the corncrake. However, even weak dependence between body size and acoustic properties of signal may be important in natural selection process. Alternatively, such a weak dependence may be observed, because receivers ignore the acoustical, formant-based cues of body size. Simultaneously, the formants might potentially provide acoustic cues to individual discrimination and could be used to census and monitoring tasks.