The breeding bird communities present in 33 isolated wetlands in the Po plain (northern Italy), ranging in size from 0.3 to 63 ha, were investigated in order to evaluate the effects of area, isolation and habitat diversity on community structure. In simple and multiple regressions, the effect of area was always preponderant (r2= 64–83%). No other variables affected the number of species nesting exclusively within the wetlands while habitat structural diversity and isolation significantly increased the explained variance of the total number of species and of other community variables. The S.L.O.S.S. (Single Large or Several Small) debate is criticized where species level studies are not carried out.