Sexual Dimorphism in the External Measurements of Black-tailed Gulls Larus crassirostris Breeding at Northern Niigata Prefecture

Abstract

ABSTRACT : Sexual differences in the external measurements of adult Black-tailed Gulls Larus crassirostris (15 males and 13 females) breeding at northern Niigata Prefecture, Japan were investigated. Males were significantly larger than females in all measurements. Black-tailed gulls at this study site had large degree of sexual difference in head region (head length, bill length, and bill depth), as did individuals from other breeding areas (Kabushima and Rishiri islands). Gender was determined using the discriminant function as follows: D = 54.099 – 0.348 HL – 1.096BD.......................................................................................(1) where HL (mm) was head length and BD (mm) was bill depth. Following this equation, birds were classified as male for D < 0 and female for D > 0. This function correctly classified 100 % of the males and females.