The influence of partial clutch depredation on duckling production

Nest depredation is the foremost cause of reproductive failure in waterfowl. Management strategies typically have focused on reducing predator contact with nests, yet the fate of nests after predators have found them has received little attention. Although nest depredation can result in complete clutch loss, nests often are only partially depredated and the remaining clutch may be successful. We investigated the prevalence of partial clutch depredation in dabbling ducks and assessed its influence on duckling production in the Suisun Marsh of California, USA, from 1998 to 2000. Partial clutch depredation by predators was common in all duck species and in all years. Overall, 37% of mallard (Anas platyrhynchos; n = 803), 37% of gadwall (A. strepera; n = 340), 22% of northern pintail (A. acuta; n = 46), 31% of cinnamon teal (A. cyanoptera; n = 16), and 1 of 2 northern shoveler nests (A. clypeata) were partially depredated. Of those nests experiencing a depredation event, 53% of mallard and 50% of gadwall nests were only partially depredated rather than completely destroyed. As a result of partial clutch depreciation, total duckling production was reduced by 10% for mallards and 9% for gadwalls. The female's decision to stay with or abandon the reduced clutch had an important influence on nest success. Mallard and gadwall females abandoned the nesting attempt after partial clutch depredation 37% and 32% of the time, respectively. However, 27% of partially depredated mallard nests and 23% of partially depredated gadwall nests were successful Egg success was 0.60 ± 0.24 (mean ± SD) for mallards and 0.53 ± 0.23 for gadwall. From 1998 to 2000, 22% of mallard and 21% of gadwall ducklings produced in our study area came from partially depredated nests. Although many cluck nests experienced partial clutch loss, they nevertheless contributed substantially to overall duckling production. Incorporating an estimate of egg success, in addition to nest success, may provide a more accurate assessment of waterfowl management strategies.