2000 Volume 48 Issue 3 Pages 183-196
Diel rhythmic activity, foraging site utilization and food habits, of the Black-crowned Night Heron (Nycticorax nycticorax) was studied in its breeding season, at colonies on the border of the Tsugaru Plain, northern Japan. During the non-breeding season they were nocturnally active, but in the breeding season they foraged both in the daytime and at night.Their main foraging sites comprised two habitats, i.e. paddyfields with irrigation ditches, and rivers. In the latter habitat, fishways and dams were most frequently utilized. The Night Herons chiefly fed on fishes, Japanese Loaches (Misgurnus anguillicaudatus) in particular. The results of comparisons of diet composition collected in the morning with that collected in the evening, and of distribution frequency of the directions in which the herons departed from the colonies and from which they returned to the colonies around sunset, suggested that the herons switched their foraging habitats between day and night; daytime in rivers and nighttime in paddyfields. The pattern of activity of the prey fishes in paddyfields is known to be nocturnal, in contrast to the diurnal activity of the river fishes. Therefore, the profitabilities of these two habitats change rhythmically in a day. This seems to be the reason why the herons switched their foraging habitats between day and night.