RESPONSES OF BREEDING DUCK POPULATIONS TO CHANGES IN FOOD-SUPPLY

We describe the main results of a monitoring study at Lake Myvatn, nor thern Iceland, begun in 1975. The aims were to find factors that limit production of young and cause changes in density of breeding ducks of several species. We estimated numbers of ducks in spring, before nest ing, numbers of ducklings produced, and numbers moulting. Chironomid a nd simuliid dipterans were monitored with window traps. In all duck sp ecies studied, production of young was correlated with food abundance. Reproductive performance determined subsequent changes in spring popu lation density of Eurasian Wigeon, Tufted Duck, Greater Scaup, Common Scoter and Harlequin Duck. The spring population of Barrow's Goldeneye apparently did not respond to variation in reproductive success. Moul ting numbers of male Tufted Duck were related to chironomid abundance, but not those of Scaup and Barrow's Goldeneye. Moulting numbers were not associated with previous reproductive output.