The choice represents a trade-off between current reproduction and future survival and reproduction. When wetland conditions are poor and ducklings have low chances of survival, a female might choose to abandon her brood, move to a more favorable location for molting, replenish her nutrient reserves, and undertake migration in better shape. Under this scenario, the female is choosing to relinquish her current reproductive effort in favor of increasing her odds of surviving to try again next breeding season, when conditions may be more favorable for brood survival.
Young ducks that have recently fledged have just grown their first set of flight feathers, so they will not undergo a molt to replace them for another year. The challenge for young birds is to build fat reserves to fuel their first migration south. Once they are capable of flight, many young ducks may move from the grassland-dominated areas where they were hatched to areas with more cropland, where high-calorie waste grain is readily available. DU biologists engaged in long-term research in the Missouri Coteau of North and South Dakota have observed that some grassland-dominated nesting areas have large numbers of duck broods in July but are nearly devoid of ducks during August. Because of changes in the birds’ physiological needs and diets, large flocks of ducks can be seen in intensively farmed areas in the early fall, even though such areas are typically poor locations for producing ducklings.
Differences in movement patterns and habitat needs between adult and young birds translate into different migration patterns in the fall. Waterfowl hunters across northern breeding areas may notice these differences during the early hunting season. In many cases, only local breeding adult females and young birds of both sexes are present in these areas early in the fall. As a result, some flocks of early-season mallards may contain very few conspicuously colored greenheads. But as the fall progresses, the young males continue with the molt of their body feathers, developing the characteristic green head, while migrations of adult drakes begin to show up and balance out the sex ratios.
Although less is known about the late-summer period than most other parts of the annual life cycle of ducks, biologists think that most of the behaviors and movements of ducks during this time are governed by the need to prepare for the rigors of migration, the demands of flight feather replacement, or both. The various strategies employed to meet these needs and demands further illustrate the adaptability of these amazing birds.
Johann Walker is a regional biologist and Dr. Scott Stephens is director of conservation planning at DU’s Great Plains Office in Bismarck, North Dakota.