Chicks perform conspicuous begging behaviour in response to the arrival of a parent. In seabirds colonies, as nests are close to each other, chicks are permanently surrounded by sound and visual stimuli produced by adult conspecifics approaching their nests. However, in spite of these conditions, black-headed gull chicks begin to vocalize as their parent approaches even before they can see it. In this paper, we report field experiments testing sound-based discrimination of parents by black-headed gull chicks. Focusing on the ‘long call’, i.e. the signal emitted by parents when coming back to the nest, we investigate here the acoustic parameters used for this recognition process. By playback experiments using modified ‘long calls’, we demonstrated that signals without amplitude modulation still elicit responses in chicks. In contrast, frequency modulation appears essential. In the frequency domain, experiments revealed that frequency analysis is precise. Chicks did not react when the frequency spectrum of parental call was shifted 20 Hz down or up. The totality of harmonics is not necessary: chicks require only two harmonics to discriminate between parents. Signal redundancy is of great significance since a minimum of four successive syllables in parental ‘long call’ are required to elicit reaction in the chick.