Foraging areas and trip duration vary with the main prey captured, in a day-foraging/night-provisioning seabird

When facing changes in the available forage fish composition and distribution, seabirds are expected to respond by switching their prey species and/or adjusting their key foraging areas. We studied chick-rearing Rhinoceros Auklets Cerorhinca monocerata at Teuri Island, Japan, across three successive years. We examined inter-annual and seasonal changes in their prey species by sampling bill-loads fed to chicks, and surveyed their foraging areas using GPS data-loggers. In 2020, auklets foraged on sand lance Ammodytes spp. in shallow coastal areas along the mainland, ~ 150 km from the colony. In 2021 however, they foraged on an alternative prey, age-0 Greenling Pleurogrammus azonus, in cool basin deep waters > 150 km away in the early season, and later switched to Herring Clupea pallasii caught in warm coastal waters. In 2022, they foraged on Anchovy Engraulis japonicus in offshore and coastal waters < 150 km away. These changes in the auklets’ foraging areas mirrored the respective habitat of the dominant prey species in each case. In turn, these changes determined their foraging trip duration: the birds made longer trips in 2021 (45.5 h) than in 2020 (30.9 h) or 2022 (22.8 h). Longer overnight trips undertaken when auklets foraged in areas more than 130–180 km away from the colony, likely decreased chick meal frequency in this day-foraging/night-provisioning species. Although they could make longer foraging trips than day-foraging/day-provisioning species, the inter-annual variability of forage fish community might result in more frequent reproductive failure under the unconventional climate regime currently in place around Japan.