We studied the diel activity patterns of presence and absence from a heronry and post-breeding roosts of ten free-living Black-crowned Night Herons Nycticorax nycticorax in Honshu, Japan. We described their diel activity patterns and examined whether the patterns of individual herons varied while living in a heronry, and whether those patterns changed as the individual lived in the heronry or in a post-breeding roost. Six tagged herons were frequently absent from their heronries during the daytime. There were four distinctive types among the observed diel activity patterns: 1) absence of individual herons observed every other day, 2) presence at the heronry occurred largely after sunrise and before sunset, 3) individuals made many short foraging trips and short visits to the heronry throughout both the day and night, 4) absent only at night (nocturnal) and individuals left the roosting place exactly at sunset every day.

The six herons that were absent from the heronry during the daytime varied their diel activity patterns while living in the heronry: for example, an individual switched from type 3 diel activity, to type 2. Three of the six herons were observed to move from a heronry to a post-breeding roost, and were found to change their behavioural pattern from being absent from the heronry during the daytime, to being absent only at night prior to, or at the time of, the move.