Addressing the question of the parental effort allocation when breeding remains central to life-history theory. This is particularly the case for monogamous species with obligate bi-parental care, where each parent should evaluate its contribution throughout the breeding season taking into account both the effort of its mate and environmental constraints. In colonial species, social environments may influence the activities of breeders. Here we compared patterns of mate effort in the Whiskered Tern Chlidonias hybrida, throughout breeding in two regions that differed greatly in colony nest density. This species builds large floating nesting platforms on aquatic plant beds. Accordingly, we explored whether nest building was sex-specific and influenced the activities of breeders throughout the season. In both regions, parental effort was indeed differentiated by sex; males invested strongly in nest-material delivery and mate provisioning during the nest-building stage, and females displayed increased investment during chick rearing. Contrary to expectations, courtship feeding and social interactions were less frequent in the region with high nest density. Our findings demonstrate that each parent clearly adjusted its reproductive effort throughout the season, whereas the contrast in social conditions between the two study regions weakly affected the activities of breeders.—Chambon, R., Latraube, F., Bretagnolle, V. & Paillisson, J.-M. (2019). Sex-specific contributions to reproduction in Whiskered Tern Chlidonias hybrida colonies of varying breeding density. Ardeola, 67: 113-125.
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Vol. 67 • No. 1