Brood Ecology of Mallards and Gadwalls Nesting on Islands in Large Reservoirs

Abstract

Duckling survival is a critical component of waterfowl recruitment rates, yet estimates of survival are seldom available for ducklings on large lakes or reservoirs attracting dense concentrations of island-nesting waterfowl. We studied brood ecology of 51 radiotagged mallard (Anas platyrhynchos) and 24 radiotagged or marked gadwall (A. strepera) females nesting on islands in 2 lakes composing the Garrison Reservoir of North Dakota, from 1985 to 1987. Duckling survival to fledging varied widely by year and reservoir, ranging from 0 to 30% for mallards and 0 to 38% for gadwalls nesting on Mallard Island in Lake Sakakawea, versus from 28 to 35% for mallards and 0 to 41% for gadwalls nesting on islands in Lake Audubon. We used logistic regression to evaluate ecological factors predicting the survival of 606 ducklings in 69 broods of these species. Duckling survival varied (all effects P < 0.001) by year, species, reservoir of origin, hatching date, initial rearing site, and several interaction terms. Survival was lower for ducklings hatched in: (1) 1985 and 1987 when reservoir water levels were low in Lake Sakakawea, (2) mallard versus gadwall broods, (3) broods of both species hatched on Mallard Island, (4) broods produced after the median hatching date of 22 June, and (5) gadwall broods moved to off-island wetlands for rearing. When water conditions allowed, mallards used shallow wetlands with ≥50% emergent cover for rearing broods and moved broods farther (x = 5.1 km ± 0.53 [SE]) (P < 0.005) to reach these wetlands than gadwalls (x = 2.5 ± 0.36) which typically used permanent wetlands with less emergent cover. Inter-related factors potentially contributing to high duckling mortality in these large reservoirs included: (1) oligotrophic water conditions, fluctuating reservoir levels, and resulting lack of food and hiding cover in emergent vegetation along shorelines; (2) long travel distances needed to reach productive brood-rearing wetlands; (3) exposure to severe weather and wave action; and (4) predation by mink (Mustela vison), California gulls (Larus californicus), and other predators.