Sex-related differences in the foraging movement of Streaked Shearwaters Calonectris leucomelas breeding on Awashima Island in the Sea of Japan

Sex-related differences in foraging habitat are common among seabirds. Streaked Shearwaters Calonectris leucomelas breeding on Awashima Island in the Sea of Japan are considered to exhibit gender differences in foraging habitat: only males cross the windy Tsugaru Strait into the Pacific Ocean. Since males are larger, with greater wing loading than females, winds are expected to increase the effect of sexual size dimorphism on their flight performance, which may determine accessibility to foraging habitats. To assess the sex-related differences in foraging movements among years in which environmental and wind conditions differed, we analyzed foraging trips of male (N= 243) and female (N= 241) Streaked Shearwaters during the chick-rearing period by using GPS loggers in 2011, 2012, and 2013. Both males and females were found to travel through the Tsugaru Strait into the Pacific Ocean, but the frequency was higher for males than for females. Nevertheless, we found that wind velocities had no effect on the probability of transiting the Tsugaru Strait. Greater wing loading requires higher energy demands for flight; therefore, males possibly needed to travel into the Pacific Ocean to feed on the energy-rich Pacific Saury Cololabis saira. In 2012, when the sea surface temperature (SST) in the Sea of Japan was the highest among the three study years, the frequency of foraging in the Pacific Ocean was similar for males and females. Shearwaters are considered to forage in association with predatory fish, the distribution of which is largely influenced by the Tsushima Warm Current migrating partially into the Tsugaru Strait. Hence, both males and females were more likely to travel into the Pacific Ocean when the Sea of Japan SST was high, generating conditional sex-related differences in foraging habitat.