The density-dependent processes that limit the colony size of colonially breeding birds such as herons and egrets remain unclear, because it is difficult to monitor colonies from the first year of their establishment, and the most previous studies have considered mixed-species colonies. In the present study, single-species colonies of the Grey Heron (Ardea cinerea) were observed from the first year of their establishment for 16 years in suburban Tokyo. Colony size increased after establishment, illustrating a saturation curve. The breeding duration (days from nest building to fledging by a pair) increased, but the number of fledglings per nest decreased, with colony size. The reproductive season in each year began earlier, and there was greater variation in the timing of individual breeding when the colony size was larger. The prolonged duration until nestling feeding by early breeders of the colony suggests that herons at the beginning of the new breeding season exist in an unsteady state with one another, likely owing to interactions with immigrant individuals. Such density-dependent interference may affect reproductive success and limit the colony size of Grey Herons. 1. Introduction Wading birds such as herons and egrets nest in colonies in relatively limited areas and at high density, often reaching hundreds or thousands of pairs [1]. Herons and egrets are usually quite large birds, and the dynamics of their population attract much attention, as estimating the number of breeding pairs is easier in colony-breeding birds than in noncolony breeders. Population fluctuations may be caused by density-dependent and density-independent processes, although the two are not mutually exclusive. Density dependence is a negative feedback between population growth rate and population density [2]. In a White Heron (Egretta alba) colony, the mean number of fledglings per nest decreased as the number of nests in the colony increased, suggesting a density-dependent response [3]. The first egg-laying date was delayed when colony size was large in a Purple Heron (Ardea purpurea) colony [4]. However, little unequivocal evidence shows that density-dependent factors regulate population fluctuations of wading birds, because intraspecific competition for breeding and colony sites appears to be unimportant in determining the reproductive success or survival in most species [5, 6]. Instead of tracking yearly changes in a single colony, intercolonial comparisons in the same year are useful to detect density-dependent processes, and such comparisons have been conducted in
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