The bittern (I xobrychus sinensis Gmelin), a migrant bird, breeds on Xinhai reed bed Fengxian, shanghai, from May to August every year. Durinng the breeding period the population density is 11.4 pairs/ha. The highest is in the irrigated reed bed with 31.1813.74 pairs/ha. The fluctu-sation in population number is remarked during the breeding period.Analyses of clumped distribution with a Poisson method show S2/X = 3.49>1, p<0.01. The nest distribution belongs to the clumped patterns.During the reproduction ...