Abstract
Great Cormorants Phalacrocorax carbo breed year round in Japan, though there are variations between colonies in the timing and duration of breeding. Males bring more nesting material than females do. The clutch consists of 2-3 eggs and both male and female parents alternate incubation duties for a month. At the Shinobazu Pond colony, Tokyo, the chicks of nests on trees fledged in 31-59 days (average 45 days). Although essentially monogamous the cormorants often mate with other partners even during the same breeding season. At the Shinobazu Pond colony, males start to breed at a younger age (1-6 years-old, average 2.1 years) than females (1-8 years-old, average 2.6 years).
