Ardea
Official journal of the Netherlands Ornithologists' Union

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Mariano-Jelicich R., Madrid E. & Favero M. (2007) Sexual dimorphism and diet segregation in the Black Skimmer Rynchops niger. ARDEA 95 (1): 115-124
Sexual segregation is a relatively common characteristic of birds and may occur at different temporal and spatial scales. Foraging differences between males and females may have been important in the evolution of size dimorphism. In this study we explore the degree of sexual segregation in the Black Skimmer Rynchops niger. We studied prey species and sizes consumed by either sex and the sex ratio distribution in the study area. We developed a multivariate tool for sex determination in Black Skimmers by using morphometric measurements from birds of known sex. Birds captured with mist-nets and museum skins were analysed. The sex of birds was determined by PCR amplification of DNA or by gonad inspection. The fish prey remains obtained from the spontaneous regurgitations of captured birds were used to estimate total length and weight of consumed fish prey. The bulk of the diet of both sexes was composed by the same species (mostly silversides Odontesthes sp), but differences between sexes in occurrence of prey species and the prey sizes were found, with male consuming larger prey than females. The sex ratio in the lagoon was strongly skewed towards females. We discuss the implications of sexual segregation for habitat and species conservation measures.


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