RESEARCH PAPERS Laboratoire de Biologie Animale, Universite  Jean Monnet, St-Etienne; CEFE±CNRS, UPR 9056, Montpellier; CNRS, NAMC MeÂcanismes de Communication, Orsay Acoustic Communication in a Black-Headed Gull Colony: How Do Chicks Identify Their Parents? Isabelle Charrier, Nicolas Mathevon, Pierre Jouventin & Thierry Aubin Charrier, I., Mathevon, N., Jouventin, P. & Aubin, T. 2001: Acoustic communication in a black- headed gull colony: how do chicks identify their parents? Ethology 107, 961Ð974. Abstract 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 conspeci®cs 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 ®eld 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 modi®ed `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 signi®cance since a minimum of four successive syllables in parental `long call' are required to elicit reaction in the chick. Corresponding author: Nicolas Mathevon, Laboratoire de Biologie Animale, Universite Jean Monnet, 42023 St-Etienne cedex 2, France. E-mail: mathevon@ univ-st-etienne.fr Ethology 107, 961Ð974 (2001) Ó 2001 Blackwell Wissenschafts-Verlag, Berlin ISSN 0179±1613 U. S. Copyright Clearance Center Code Statement: 0179-1613/2001/10711±0961$15.00/0 www.blackwell.de/synergy