Alternative Nest Strategies: Parasitism
Posted on 07/07/2015
Many female ducks attempt to increase their reproductive output and avoid nest predators by laying eggs in another female’s nest, which is known as nest parasitism.
This reproductive strategy is common where nests are easy to find — cavity nesters such as wood ducks, over-water nesting ducks such as canvasbacks, and ducks that nest on islands. Nest parasitism is a common behavior of redheads. This spring in southwest Manitoba, nearly 90 percent of canvasback nests monitored by Delta Waterfowl researchers contained redhead eggs.
Parasitism is obviously a great deal for parasitic hens: They not only avoid predators, but also have their eggs hatched and ducklings raised by another hen.
Host females — hens incubating the eggs — can hatch far more eggs than they lay, and ducklings feed themselves, so parasitism usually is not terribly costly for most host species. However, for over-water nesting canvasbacks parasitized by redheads, there can be a substantial cost when eggs are pushed out of the nest by the scuffling of the hens.
The parasitic redhead bullies the host canvasback, lays her egg, and then leaves the nest. The hen canvasback is left to incubate the eggs and care for the ducklings.