By J. Dale James, Ph.D.
Waterfowl have developed many behavioral adaptations that occur throughout their life cycle, from the selection of mates and nest sites to migration and foraging strategies. All these adaptations in some way help increase the birds' survival and sustain their populations. One of the most interesting behavioral adaptations among waterfowl, which occurs during the egg-laying portion of the breeding cycle, is brood parasitism.
For both waterfowl and humans, raising young is costly in time and energy. Brood parasitism is the act in which birds avoid many of these parental costs by laying eggs in the nests of other birds, who then incubate and raise the parasitic offspring along with their own. While brood parasitism is among the rarest reproductive strategies in the bird world, a surprising number of waterfowl species use this strategy. Unlike the mockingbirds or blue jays in your backyard, ducks do not defend the immediate vicinity of their nests during the laying period, which allows easier access to nest sites by parasitic females.
There are two forms of this behavior: conspecific (or intraspecific) brood parasitism, in which eggs are laid in the nests of the same species, and interspecific brood parasitism, in which eggs are laid in the nests of other species. Some species do both, and so-called "dual strategists" may lay eggs in other birds' nests and then lay and incubate eggs in their own nests.
Conspecific brood parasitism, often referred to as dump nesting, is the most common form of this behavior in waterfowl. More than one-third of the 200 avian species that routinely use this strategy are waterfowl. It's especially prevalent among North America's cavity nesters, such as wood ducks, goldeneyes, buffleheads, mergansers, and black-bellied whistling ducks. Rates of brood parasitism reported among these species range from 5 to 8 percent in buffleheads to greater than 90 percent in certain populations of wood ducks and black-bellied whistling ducks.
Interspecific brood parasitism is much less common among waterfowl. In North America, the species best known for this strategy is the redhead. Researchers on Manitoba's Delta Marsh found that in some years more than 90 percent of canvasback nests were parasitized by redheads and upwards of 50 percent of the eggs present in canvasback nests were those of redheads. But redheads target other nesting waterfowl as well, including both diving and dabbling ducks. Remarkably, redheads raised by other species are aware of their identity and join flocks of their own kind after fledging.
At high rates, brood parasitism can have negative effects on host species. Altercations at the nest between host and parasitic hens can result in damaged eggs, and, in extreme cases, nest abandonment. In addition, incubation efficiencies may be reduced if too many eggs are laid in a single nest. At low levels, however, this behavior can be beneficial to both host and parasite. In what is known as the "selfish herd" effect, brood parasitism may benefit hosts by increasing overall duckling production, diluting the risk of predation of the host's young through sheer numbers. In addition, among ducks with high nest-site fidelity, several closely related females (i.e., sisters, mothers, and daughters) may regularly nest in close proximity to each other. In these cases, the costs of brood parasitism would be reduced as the hosts would be rearing the young of closely related individuals who carry the same genes.
Regardless of what drives this behavior, it's easy to see why some waterfowl species have adopted brood parasitism as a reproductive strategy. If someone else is doing the incubation and brood rearing, hens can devote more energy to staying alive or, in the case of dual strategists, producing and laying their own optimally sized clutches. By laying eggs in different nests, these females reduce the chances of losing all their reproductive effort if their own nest is destroyed, and in environments with unpredictable habitat availability, they can freely seek out needed foraging resources and not be constrained by having to watch their ducklings. While these birds might not win any awards for their parenting skills, what ultimately matters in nature is the perpetuation of their genes, by any means.
Dr. J. Dale James is director of conservation planning in Ducks Unlimited's Southern Region.
Did You Know?
- Brood parasitism is believed to be common among cavity nesters in part because the birds suffer from a shortage of suitable nesting sites, which can result in several females laying eggs in the same nest. However, studied populations of common goldeneyes and black-bellied whistling ducks indicate that more than 50 percent of nest boxes remain unused in some locales, so additional explanations likely exist for this behavior.
- Placing nest boxes in conspicuous locations, such as on the edge of a pond, or in groups can increase the incidence of brood parasitism among wood ducks and other cavity-nesting waterfowl. This occurs because parasitic hens can easily observe host birds coming and going from these man-made nest sites. Thus, nest boxes should be placed in less visible locations—such as in wooded areas—near suitable wetland habitats.
- In south Texas, researchers discovered that due to dump nesting one black-bellied whistling duck nest contained 101 eggs laid by multiple females. Amazingly, a single hen incubated and hatched 38 of those eggs.
- Hooded mergansers, which are well-known brood parasites, have eggs that closely resemble billiard balls in size and thickness. Initially, waterfowl biologists believed that the eggs' hard shells offered protection from host females, who could try to break them if they were discovered in their nest. However, most cavity-nesting waterfowl have relatively thick-shelled eggs, presumably because they are laid in confined spaces where they are more likely to be jostled during incubation.
- Researchers have found that rates of nest parasitism are lower among smaller ducks that nest in cavities, such as buffleheads. This is likely because smaller nest cavities are more numerous in the environment, giving these species more potential nesting opportunities, especially in secluded habitats.
Black-bellied whistling ducks regularly parasitize the nests of members of their own species.
Brood Parasitism or Bust
Some bird species, known as obligate brood parasites, forgo nesting altogether and rely entirely on other species to incubate their eggs. Yellow-billed cuckoos and cowbirds are well-known examples of this behavior. Among waterfowl, only one example of a true obligate parasite exists: the black-headed duck. This South American species chooses a wide range of hosts to incubate its eggs but is particularly fond of the rosy-billed pochard. Black-headed ducks also parasitize the nests of fulvous whistling ducks, coots, and even herons, ibises, gulls, and snail kites. Interestingly, the parasitic offspring of the black-headed duck do no harm to their hosts, as the ducklings are completely independent and fledge soon after hatching.
The black-headed duck relies entirely on other birds to incubate its eggs.