Regular surveys of Great Blue Heron (Ardea herodias) colonies in Quebec, conducted between 1977 and 2001, resulted in the discovery of 705 colonies, mostly in the western part of the province. While single breeding pairs were occasionally found (9% of Quebec "heronries"), the colonies were generally composed of fewer than 16 breeding pairs, with sometimes over 50 (18 known colonies), as was the case on several islands in the St. Lawrence River. Close to one-third (1919 of the 6481 active nests) of the Quebec Great Blue Heron population was associated with heronries in the St. Lawrence system. Half of the St. Lawrence heronries that were active during two adjacent survey periods (1975-1983 and 1984-1990, or 1984-1990 and 1991-2001) increased in size between periods, while one-third of the colonies decreased in size. One-sixth of the colonies remained the same size during the study period. Great Blue Herons periodically abandon their colonies and settle elsewhere. Our data show that 50% of known Quebec colonies (n = 377 colonies visited more than once) were abandoned after being discovered as active colonies, and, on average, all nests in colonies that are abandoned disappear 7 years after discovery of the colony. On the basis of thorough inventories conducted on the St. Lawrence, we found that all nest platforms in 14 of the 70 colony sites had disappeared in less than 25 years. Since 1977, when regular surveys began, 50-90% of breeding attempts have been successful, yielding an average of over 2.2 chicks per pair. This productivity rate suggests that the Quebec population of Great Blue Herons may be increasing and may explain the recent expansion in its range. Following our 25 years of investigation on the Great Blue Heron in the province, we estimate the current late-summer Great Blue Heron population of Quebec at some 27 000 individuals (6500 pairs producing 2.2 young per pair per year).--Publisher's description.