POPULATION-DYNAMICS OF THE BARNACLE GOOSE BRANTA-LEUCOPSIS IN THE BALTIC AREA - DENSITY-DEPENDENT EFFECTS ON REPRODUCTION

1. The three largest breeding colonies of barnacle geese Branta leucop sis in the Baltic area were studied from their natural establishments in 1971, 1982 and 1987, respectively, to 1993. The number of breeding pairs increased from one pair in 1971 to 1550, 225 and 150 pairs in th e largest, second largest, and third largest colonies, respectively, i n 1993. 2. Data on clutch size and number of hatched and fledged young were collected from the beginning or the middle of the 1980s. In the largest colony most data on reproductive success were obtained from ob servations of individually marked geese. 3. Mean clutch size differed among years and colonies. However, there was no trend for decreasing m ean clutch size with increasing density in any of the colonies. Hatchi ng success did not decrease with increasing density in the largest col ony. 4. The production of fledged young per breeding pair declined dra stically in the largest colony as numbers of breeding pairs rose. This decline was mainly caused by an increasing proportion of the breeding pairs failing to produce any fledged young at all. 5. The relative co ntribution of the smaller colonies to the total production of fledged young in the Baltic area increased considerably over the study years. In 1993, colony 2 hosted only about 13% of the number of breeding pair s in colonies 1 and 2 combined, but it produced about 38% of the fledg ed young. 6. Our results point out the need to study populations of co lonially nesting geese on a colony by colony basis in order to detect density-dependent effects on reproduction.