(1) In two colonies of Common Cormorants in the Shimokita Peninsula, Aomori Pref., breeding activity and breeding success were investigated from April to September 1981. One of the colonies (Ichiyanagi colony) consisted of 43 nests and the other (Yamabezawa colony) 34 nests.
(2) There was no close relationship between preferred nest site and nesting time in both colonies. The nests were not uniformly distributed in the colonies and each colony was roughly divided into dense and sparse parts.
(3) The proportion of nests with the young was significantly higher in the dense part than in the sparse part at the Ichiyanagi colony.
(4) The number of fledglings per nest was significantly higher in the earier breeding season (May to June) than in the late season (July to September) in both colonies
