Seasonal changes of the at‐sea distribution and food provisioning in rhinoceros auklets

Abstract

AbstractCentral‐place foraging seabirds increase food‐loads and decrease meal frequency when they forage in areas that are distant from the breeding colony. In 2001–2002, we studied the seasonal changes in at‐sea distribution, food‐load mass, meal frequency, and fledging mass in rhinoceros auklets (Cerorhinca monocerata), which forage in coastal waters during the day and feed their chicks at night. In both years, greater numbers of auklets were observed flying in northern waters that are more distant from the colony in June (65 km) and July (65–66 km) than in May (38–47 km). In July of both years, many auklets flew northward across the transect set 65–120 km north of the colony at sunrise; the birds returned south again at sunset, indicating that they foraged in waters outside the study area. This seasonal northward movement of the foraging area may reflect the migration of their main prey item, the Japanese anchovy (Engraulis japonicus), which move with the Tsushima Warm Current flowing from the southern Sea of Japan. Food‐load mass did not increase seasonally. In both years, the estimated daily meal frequency was lower in July than in May or June, partly because of the increased foraging distance in July. Late‐hatched chicks also displayed lighter fledging masses than early chicks in both years. We suggest that late breeders are required to forage at great distances for longer periods, which may result in decreased meal frequency and lighter fledging mass of their chicks.