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When suitable nesting habitat is rare, birds may have to share it with heterospecific individuals with similar nesting requirements. The resulting species mosaic may in turn affect how breeding birds communicate vocally with each other. For instance, they may modify their vocalizations if the ambient noise produced by heterospecifics interferes with their own vocalizations. The colonies of Gentoo Penguins (Pygoscelis papua ellsworthi) are interesting to examine whether such a vocal variation occurs in colonial seabirds, as these birds frequently breed in the same colony as Adélie (P. adeliae) or Chinstrap (P. antarcticus) penguins. We investigated whether the presence of congeneric penguins breeding in the same colony as Gentoo Penguins affected the acoustic characteristics of their most common vocalization, the ecstatic display call. Based on vocalizations recorded in 23 breeding colonies along the Antarctic Peninsula during 3 field seasons, we found that the frequency of ecstatic display calls of Gentoo Penguins was consistently lower (average exhale phrase ~460 Hz lower, average inhale phrase ~370 Hz lower) as well as decreased in energy distribution when they bred in mixed colonies with Adélie Penguins than when they bred only in proximity to conspecifics. In contrast, the frequency of both inhale and exhale phrases was unaffected by the presence of Chinstrap Penguins, potentially due to the already greater frequency difference between these 2 species. The apparent vocal plasticity, in mixed colonies with Adélie Penguins, likely allows Gentoo Penguins to adjust the quality of their vocalizations depending on the acoustic space available in their colonies, and hence possibly enhance signal transmission between conspecifics. However, whether this acoustic adjustment is sufficient to increase the detectability of vocalizations and localization of individuals has yet to be determined. [ABSTRACT FROM AUTHOR]