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AGE AND SEX DETERMINATION IN BLACK SKIMMER CHICKS
Black Skimmer (Rynchops niger) Science Article 4
Post-natal development was studied in 40 southern California Black Skimmer (Rynchops niger) chicks in order to develop a reliable method of aging and sexing pre-flying young in this sexually dimorphic species. Ages of chicks were determined from hatching dates and were calculated also from wing chord measurements. At fledging, male and female chicks exhibited the same degree of sexual size dimorphism previously demonstrated in adult birds. Sex of chicks was determined after day 23 when differences in body weight became significant. At day 24 or older, chicks were classified as male if their weight exceeded 320 g, and as female if less than 300 g. This technique should allow a closer examination of sexually related aspects of Black Skimmer demography during the pre-reproductive period.
WILLIAM A. SCHEW CHARLES T. COLLINS, Journal of Field Ornithology: Vol. 61, No. 2, Spring, 1990
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