The Yellow Bittern is a small ardeid bird, foraging mainly on small fishes and nesting in emergent vegetation. In order to study its foraging and breeding behaviors, we observed its foraging activity with a telescope and monitored its breeding stages by using a video camera set close to its nest in a small irrigation pond during its breeding season. We also measured seasonal and diel changes of DO (dissolved oxygen) in the pond, and examined the fish fauna of the pond and of the adjacent paddy fields.

From the onset of draining of the paddy fields in July, bitterns exhibited frequent flying to the paddy fields, but ceased such behavior a few days after re-introduction of water. The ditch adjacent to the paddy fields retained some water during the draining period, and was inhabited by loaches.

In the pond, on the other hand, the most abundant small fish were Stone Morokos, a main prey. The average prey size caught by the bitterns decreased in early July, which corresponded to the recruitment period of the young-of-the-year Stone Morokos.

The bitterns used floating-leaved plants as footholds for foraging as season progressed, particularly in later daytime when DO elevated. These changes in their foraging behavior were related to the rising behavior of Stone Morokos to the water surface caused by hypoxia developed by the floating-leaved plants.