Daytime and nighttime activity at a breeding colony of Great Blue Herons in a nontidal environment

Abstract

It is generally admitted that in coastal areas, herons of the genus Ardea adjust their foraging time according to the tidal cycle. However, to what extent do tides control the herons' daily rhythm of activity? To answer this question, we present the day and night activity patterns of Great Blue Herons (Ardea herodias) arriving to feed their young at a heronry located in a nontidal environment in southern Quebec. Herons were about half as active at night as during the day. Therefore, although significantly less than diurnal activity, nocturnal activity was not negligible, and consequently the tide cycle is not the only factor controlling the daily rhythm of the herons' activity. Those breeding pairs that were most active during the day were no more or less active at night. Diurnal activity was more closely correlated with the number of young that fledged than was nocturnal activity. Thus, night activity was not necessarily important for the survival of young herons, but it could be explained by other factors such as the greater availability of certain prey at night.