A female Hooded Merganser approaches a nest box while a pair of Wood Ducks looks on. (Photo by Gary Shackelford)
Wood Ducks and Hooded Mergansers are two of the seven species of North American ducks that regularly nest in natural cavities. They also readily accept nest boxes that have been placed near a body of water. From many years of observing the behavior of these birds from a blind at one of the ponds at Fair Meadows Sanctuary, one fact has been made abundantly clear: there is keen competition for nesting sites, both in artificial structures and in tree cavities.
Fledgling Wood Duck (left) and Hooded Merganser (right) simultaneously leave their nest box. (Photo by Gary Shackelford)
The strategy of laying their eggs in other birds’ nests, called brood or nest parasitism, has been documented in both of these duck species. Conspecific nest parasitism (laying eggs in a nest of the same species), also known as egg dumping, is common among Wood Ducks. Signs of this behavior in Wood Ducks include laying rates of more than one egg per day and clutch size of 16 or more eggs. Conspecific nest parasitism also occurs with Hooded Mergansers, but it is less well studied. Interspecific nest parasitism (laying eggs in a nest of a different species) also occurs in these two species. Each species is known to parasitize, and to be parasitized by, the other. Nest boxes are more frequently parasitized than nests in natural cavities, presumably because nest boxes are placed in more visible locations. On numerous occasions, I have seen female Wood Ducks and Hooded Mergansers entering the same nest box, and on one occasion, I observed a female Wood Duck removing a Hooded Merganser egg and carrying it down to the pond in her bill. One year, at a friend’s property, I observed a fledgling Wood Duck and several fledgling Hooded Mergansers leaving a nest box concurrently, confirming successful hatching of eggs of both species in the same nest. At that particular nest, the parent bird was a Hooded Merganser.
European Starlings and some species of native songbirds may also attempt to take over nest boxes. I have observed a female Wood Duck removing a starling egg from her nest box, and I have seen Great Crested Flycatchers enter a nest box that was being used by both a Hooded Merganser and a Wood Duck. (Great Crested Flycatchers seem to appreciate the availability of cavities provided by nest boxes throughout the landscape. This past nesting season, a pair nested in one of our kestrel nest boxes in the prairie.) Once I even found a squirrel nest in a nest box!
Female Wood Duck and Northern Flicker in a standoff at a nest cavity in an aspen tree. (Photo by Gary Shackelford)
In addition to the rivalry at nest boxes, Wood Ducks and other cavity nesters compete for natural tree cavities. On one occasion, I witnessed a standoff between a female Wood Duck and a female Northern Flicker at a cavity in an aspen tree. The Wood Duck was perched on a branch near the cavity, but the flicker had the advantage because she was stationed within the cavity, and the Wood Duck eventually departed without entering.
By increasing the availability of nesting cavities—through erecting nest boxes or leaving tree snags standing—we are able to witness the fascinating nesting strategies of an incredible diversity of birds who need these habitats to thrive.
Written by Gary Shackelford, Fair Meadows Sanctuary manager
