by Johann Walker & Scott Stephens, DU Magazine: July/August 2007
As summer wanes, many people are busy with last-minute vacation plans, and some are beginning to prepare for the upcoming fall hunting season. Late summer is also busy for ducks. In July, August, and September, post-breeding adult birds and newly fledged young ducks undertake a host of activities that prepare them for fall migration.
The late-summer period between breeding and fall migration is not as well studied as the nesting and brood-rearing periods. Nevertheless, biologists know plenty about the habitat and nutritional requirements of ducks and their efforts to fulfill those needs. In late summer, there are important differences in habitat use and movement among adult males, adult females, and young birds.
Once most nesting females begin incubation and renesting activity wanes, adult drakes leave breeding areas for molting locations. Because of their early start, they are able to make long-distance molt migrations. They gather on permanent wetlands that have both emergent vegetation (where a flightless duck can hide from predators) and abundant, high-protein food (to facilitate rapid regrowth of flight feathers). These areas allow adult males to safely make it through one of their most vulnerable periods. Drakes often congregate and molt in the Canadian parklands or boreal forest far from prairie breeding areas. This interdependence among breeding and molting areas underscores the importance of maintaining diverse wetland communities across the prairie/parklands and boreal forest.
If adult females fail to hatch a nest relatively early in the breeding season, they may forego further nesting attempts and make molt migrations similar to those of adult males. But adult females that lose a nest or brood later in the breeding season, as well as those that successfully hatch a nest and raise a brood, are likely to molt on or near breeding wetlands. Because these females have been occupied with breeding activities later into the summer, they don't have enough time or energy reserves to make regional movements to molting areas before falling temperatures force them to migrate south. Regardless of whether adult females breed successfully, they need the same types of molting habitat as adult males: wetlands with extensive stands of emergent vegetation and abundant high-protein food.
Hens that successfully raise a brood must contend with complex time constraints. To survive until the next breeding season, they must molt and must replenish nutrient reserves before migrating south. As a result, some may abandon their brood when the ducklings are close to fledging. A hen is more likely to abandon her brood early when wetland conditions are either very good or very poor, when her body condition is poor, and when few ducklings remain in the brood. She must weigh the costs of staying with her brood longer to increase their chance of survival against the benefits of safely molting her flight feathers and recovering her body condition in time for a successful migration. 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.