Illustration by Adelaide Tyrol
If you peek into a wood duck nesting box during the breeding cycle, you might find 10 to 11 eggs, which is the bird’s normal clutch size. But you might also stumble upon a box overflowing with as many as 30 eggs. How, you might ask, can one duck lay and care for so many eggs? The answer is: she can’t.
These huge piles of eggs result from intraspecific brood parasitism, otherwise known as egg dumping. This is when a bird lays eggs in a nest that does not belong to her. Waterfowl – and wood ducks in particular – often engage in this behavior.
“You’ve no doubt heard the expression, ‘don’t put all your eggs in one basket’? Well, wood ducks take that literally,” explained Robyn Bailey, NestWatch Project Leader at the Cornell Lab of Ornithology. “It is a low-effort way to try to get a few more of your own young to survive, and it frequently works – females will take care of the ‘foster eggs.’”
Wood ducks (Aix sponsa) pair up in January, and couples generally make the trip back to their breeding range together in March. The ideal nesting site is a cavity in a tree or a nesting box close to water. The female inspects the site, and if she decides it is to her liking, she lines it with feathers from her breast. She then gets busy producing eggs to fill her nest and, often, those of her closest neighbors.
Egg dumping is most likely to occur when nests are in close proximity to one another. “It’s not scarcity of good nest sites, but ease of finding another host nest, that influences the behavior,” said Bailey. This can make nesting boxes good candidates for egg dumping, if they are placed close together – especially if they’re in open areas that are easy for competing hens to find. But egg dumping is in no way restricted to nesting boxes. In fact, in a recent research survey of nests, 85 percent of nests in natural cavities were parasitized, compared to about 44 percent of nest boxes. We tend to hear more about egg dumping in nesting boxes, said Bailey, because they are more easily studied.
According to Bailey, there is no evidence to suggest that the ducks know which eggs are their own and which belong to others. Hens don’t even appear to be too choosy about species; other cavity-nesting ducks, such as hooded mergansers, will sometimes dump eggs into the nests of wood ducks, and the hens will raise these young as their own.
A female wood duck whose nest has been parasitized makes no effort to get rid of the extras, though she will sometimes lay fewer of her own eggs to keep the clutch size manageable. A single female can incubate at least 20 eggs at a time.
Problems arise when the number of eggs grows beyond a couple of dozen. At that point the clutch becomes too much for one bird to manage, and so she abandons the nest. Sometimes a bird will stick with an oversized clutch, but the hatch rate will be low.
While excessive brood parasitism may force a female wood duck to abandon her nest, all is not lost. She may choose another nesting site and start over again. Wood ducks also happen to be the only North American ducks that regularly produce two broods in one year, and egg dumping tends to occur more frequently early in the season.
The day after the chicks hatch, they jump from their nest - sometimes from as high as 50 feet - and make their way to water. The female wood duck calls them to her, oblivious as to which ones are hers and which are not, interested only in their survival.
If you’re interested in monitoring wood duck nests, and would like to learn more about Cornell’s NestWatch project, click here.
Carolyn Lorié lives with her rescue dog and very large cat in Thetford, Vermont.