Sex Determination of Great Cormorants (Phalacrocorax carbo sinensis) using Morphometric Measurements

Great Cormorant (Phalacrocorax carbo) is monomorphic in plumage such that sexes cannot be separated by plumage characteristics. In contrast, it displays sexual size dimorphism, with males generally being larger than females. Sexual dimorphism and variability in size of the continental Great Cormorant (P. c. sinensis) was studied in Greece to develop useful sexing techniques using morphometric measurements. Body mass, wing, culmen, and tarsus length of 81 birds controlled under license were measured during the wintering season in 1999-2002. The sex of each bird was determined by dissection and gonadal inspection. Forward stepwise discriminant analyses were performed to provide reliable functions that would enable the prediction of sex of a bird. Differences in size between adult and juvenile birds were not significant in both sexes, whereas males were larger than females in all measurements. Body mass (19.2%) and culmen length (11.1%) were the most dimorphic variables of those looked at followed by tarsus (6.5%) and wing (6.1%) length. Although wing length was the least dimorphic variable, it also displayed the lowest coefficient of variation (2.3%) thus being the best single measurement for separating the sexes. Three discriminant functions that correctly classified 92.6-95.1% of Great Cormorants of this sample were produced. These functions were reliable (similar accuracy for discriminant analysis and jackknife validation) and seasonally unbiased, as body mass was excluded from the analyses. The function including wing and culmen length as variables showed somewhat lower accuracy when tested with a new sample from The Netherlands suggesting that the obtained functions should be applied with caution to other populations, especially within the area of overlap between the sexes, unless inter-population sources of variation (e.g., geographic variation, hybridization, inter-observer bias) are sufficiently understood.