Density dependent survival of mallard ducklings in boreal lakes: a field experiment

From earlier field studies and experiments we have found that the nutrient status of lakes in the boreal region of northern Europe might influence the production of ducklings among dabbling ducks. For ducks, this might be important in the choice of breeding strategy. In this study we created a 2-year cross-over field experiment in northern Sweden where mallard hens and broods (10 ducklings) were introduced on 10 oligotrophic lakes (i.e. poor lakes) in two different densities; 4 hens with broods (high density) on 5 lakes and 1 hen with brood (low density) on the remaining 5 lakes. The adult ducks were wild birds, caught and brought to a game farm where they hatched their eggs. Within a few days after hatching they were released on the lakes in the beginning of June when wild mallards have their hatching peak. After introductions the survival of broods, ducklings and hens were monitored daily the first 12 days and then every third day until day 24. The second year there was a switch of treatments in the lakes. The results after one year indicate that the survival rates of broods, ducklings and hens are density dependent. In lakes with high densities of introduced mallards the survival rates was higher compared to low density lakes.