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10 Years Dynamics of Heronries

FORKTAIL 29 (2013): 71–77

Long-term population dynamics of colonial herons and egrets are well documented in Europe and the USA, but not in East and South-East Asia. Here the population dynamics of mixed-species colonies from 2002 to 2011 in Ibaraki prefecture, east Kanto, Japan, are reported. From censuses based on a combination of aerial and ground surveys, the number of breeding colonies was found to vary from 15 to 20. The population dynamics of Great Egret Casmerodius albus and Black-crowned Night Heron Nycticorax nycticorax remained relatively constant, while Grey Heron Ardea cinerea and Intermediate Egret Mesophoyx intermedia increased, but Little Egret Egretta garzetta and Cattle Egret Bubulcus ibis decreased. When data for the six species were combined, the sum of their populations was almost constant, but variation increased in colony size, species composition ratio and the number of years that individual colonies existed. The population of colonies typically ranged from 200 to 2,000 individuals up to 2004, but smaller (under 200 individuals) and larger (over 2,000 individuals) colonies appeared after 2006. Increased variation in the number of consecutive years colonies existed was closely related to increased variation in colony size. Increased variation in species composition ratios in colonies was not a by-product of the increased variation in colony size, and the occurrence of colonies dominated by Grey Heron, Intermediate Egret or Black-crowned Night Heron after 2006 played an important role in the structural changes of mixed-species colonies.