Use of Morphometric Measurements of Photographs of a Sexually Dimorphic Bird to Determine Sex

This study investigated if morphological measurements made from digital photographs of banded adult Black Skimmers (Rynchops niger), using alphanumeric field readable leg bands as a scale reference, can be used to determine their sex. Black Skimmers are known for their extreme sexual dimorophism and make an excellent study subject. Measurements made in the field using calipers to determine length of the exposed culmen of skimmer chicks were significantly correlated to measurements made by three technicians using open-source software (ImageJ) measuring the culmen from photographs, with little variability among technicians. To determine sex, ImageJ was used to measure the culmen, bill depth at base, lower mandible and head + culmen lengths from photographs of 24 known sex adult skimmers (11 female, 13 male). Males were significantly larger than females for all four measurements and a stepwise Discriminant Function Analysis (DFA) using all 4 measurements retained exposed culmen and bill depth as the best predictor variables. This model correctly classified 100% of known sex males and females. Using the DFA equation, 40 additional banded skimmers were classified, and it was determined that 46 of the banded skimmers that were photographed as adults were male and 18 were female. Using a field readable band as a reference scale for other measurements may prove useful for research on other sexually dimorphic species that are banded and later photographed in the wild.