Sexual differences in the external measurements of Black-tailed Gulls breeding on Rishiri Island, Japan

Abstract

Sex ratio is an important parameter to consider in ecological and conservation seabirds studies (Weimerskirch & Jouventin 1987). Visually assessing with certainty the sex of live seabirds, however, may be an impossible task when no obvious differences in plumage or body size exist between the two sexes. Several previous studies of Laridae species have shown that males are significantly larger than females (external measurements) and discriminant functions using measurements sex Laridae with great reliability (Shugart 1977; Ryder 1978; Fox et al. 1981; Schnell et al. 1985; Coulson et al. 1983; Evans et al. 1993; Bosch 1996; Rodriguez et al. 1996; Palomares et al. 1997). The Black-tailed Gull Larus crassirostris is a medium-sized gull, endemic to the northwest Pacific, breeding extensively in coastal regions around the Japanese archipelago. Although Black-tailed Gulls are one of the most common seabird species in Japan, little is known about their biology or morphology. Similarly to other Laridae species, Black-tailed Gulls have no sexual differences in their plumage or col- oration. This study is aimed at describing the external measurements of Black-tailed Gulls and determining a discriminant function using the measurements to fa- cilitate the sexing of the gulls in the field.