Regulation of an isolated population of Great Cormorant
The great cormorant Phalacorax carbo is a cosmopolitan fish-eating bird with an isolated resident population (subspecies carbo) in Iceland. Today it breeds almost exclusively on low islets on the west coast where there are extensive tracts of shallow sea (3000 km2 of a total of 6900km2 <20m deep). Within Iceland, the cormorant is a partial migrant spending March-August on the breeding grounds and dispersing in September-February. We censussed the breeding population three times in 1975-90 and annually from 1994 onwards. We used low-level aerial photography to count cormorant nests and broods, and ground surveys to study age composition in September and February. We estimated fecundity and survival from these observations. Environmental influences, mainly food and disturbance, were also studied. The breeding population (2400-5200 nests) was regulated in a density dependent manner on the breeding grounds. Following a synchronous decline in 1990-1994, the population increased in 1994-2013 by 3.7% annually. The annual rate of change in numbers differed between the two main breeding regions, the bays Faxaflói and Breiðafjörður. We speculate that the increase was caused by high survival to the age of recruitment and was at first absorbed by persistent colonies but later on by colonization of new or long abandoned sites. Agonistic behaviour of established territory holders and decisions by groups of recruiting individuals appear to be the key internal factors adjusting the density of the cormorant population to the two most important resources: food and a secure breeding site.