SEXUAL DIMORPHISM IN THE CORMORANT Phalacrocorax carbo sinensis: POSSIBLE IMPLICATIONS FOR DIFFERENCES IN STRUCTURAL SIZE

A sample of Cormorants which accidentily had been drowned in fishermen's gear was used to study differences in morphological characters and diet between males and females. Males were significantly larger than females in all body dimensions, whereas size of juveniles and adults were the same. Discriminant analyses applied on length of body, wing and culmen, and bill depth, achieved highest classification rates by a function which makes use of body length, wing length and bill depth. Bill depth and wing length showed best segregation between both sexes in all functions. Main fish species taken were Ruffe, Perch and Smelt. No difference was found in the proportion of each fish species in the diet of males and females. For Ruffe, Perch, Eel and Smelt, only in the latter two species significant larger individuals were taken by males. Differences in structural size are discussed with respect to individual hunting performance of males and females. Rijkswaterstaat Directorate Flevoland, P.O. Box 600, NL-8200 AP, Lelystad, The Netherlands.