Ducks Sitting in the Trees

PUBLIC DOMAIN
In late September I surprised three wood ducks (Aix sponsa) on the pond at Langenau Forest. I walk the dog up there regularly but had never seen this species there before. I assumed they were dispersing from a nearby beaver swamp before migrating. They may not be going very far; this species will remain as far north as southern New Hampshire, if it can find open water through the colder months.

This small group consisted of an adult male, an adult female and a juvenile. The male was back in his familiar psychedelic finery, having gone through his “prealternate molt” in the summer, which makes him resemble the female. The molt cycles of this species are especially complex, with males and females undergoing their own cycles with different timing. The females, for example, will undergo their prealternate molt prior to breeding, in order to look adaptively drab through that period.

The birds that I saw did not fly away, even though my dog ran down to the shoreline to chase them. Instead, they all swam away across the pond, which made me suspect they may have been in another part of the cycle. While the earlier prealternate molt is one of body feathers, the “definitive prebasic molt” of late summer and early fall exchanges the wing feathers and makes the birds flightless for at least three weeks.

A. sponsa shares its genus with only one other species, A. galericulata, the mandarin duck of eastern Asia. The genus was part of a small tribe, the Cairinini or perching ducks. The systematics of ducks is always in flux, and “perching ducks” has been found to be paraphyletic, meaning the group has more than one ancestral species and is therefore an artificial grouping. They were thought to be related because of their shared un-duck-like habit of regularly perching high in trees. This is now understood to be an example of convergent evolution.

The wood duck is a cavity nester. Hence the popularity and success of nesting boxes. This is not a unique ecology. Among North American ducks, the common and hooded mergansers are both cavity nesters (although the red-breasted is not), as is the black-bellied whistling duck of the Gulf coast. None

of these ducks excavates their own nest cavity. They sometimes occupy the abandoned sites of the pileated woodpecker, but the cavities used are usually those that form where a branch has broken from the trunk and heart rot has formed a hole. The nests are on average about 25 feet from the ground.

Many duck species breed in marshes and on ponds out on the prairie and tundra, where there are no trees. Wood ducks prefer to nest in swamps and along streams through wooded areas. According to studies cited at birdsoftheworld.org, the incubating female prefers to be within 1,500 meters of flooded habitat that is 50-75% cover versus open water. It has been observed that survival of young is best in what the U.S. Fish & Wildlife classification calls a scrub-shrub wetland. This habitat consists predominantly of woody plants adapted to places that are flooded for all or part of the year.

But wood ducks are abundant and widespread, so they are not limited to this type of habitat. They will also forage in marshes (predominantly herbaceous emergent vegetation) and flooded bottomland forests. An important ingredient is that the area remain flooded through the nesting season, which extends from April to early July. The conditions favored by the brooding female are also best for her fledgling brood.

Although the male accompanies the female during the search for a nest site, he does not play any role in selecting the site nor does he incubate the eggs or in any way aid in the rearing the brood. Courtship displays are seen as often in fall as spring in this species, and they remain paired while the female is brooding, but not later in the summer, when the male is in his drab “eclipse” plumage.

Like all waterfowl, wood duck populations were greatly reduced by unregulated hunting in the 19th century. By the turn of the 20th century, it was largely eliminated from regions near settlements, but apparently survived in good numbers in remote swamps, because it recovered well after the imposition of the Migratory Bird Act in 1918. In addition to hunting, however, reduction in the extent of its wetland habitat and extensive cutting of mature forests along waterways limited its population growth. Hence the introduction of nest boxes. The eastern deciduous forest recovered in the late 20th century and has continued to mature in the 21st. Trees old enough to lose major limbs and develop rotted cavities are becoming more common.

Even so, wood ducks sometimes have a tough time finding nest sites and may end up in a cavity hundreds of meters from water. The scarcity of nesting sites also leads to nest parasitism. It is not uncommon for wood ducks to parasitize each other and, in the Midwest in particular, up to 37% of wood duck nests have been found to include the eggs of a hooded merganser. It is a successful strategy, as an average of 15 eggs hatch in the so-called “dump nests,” while the average is 10 in a non-parasitized nest. Eggs and nestlings are eaten by black rat snakes, raccoons, and squirrels. Only 20-30% of the brood survives its first 30 days with most of the mortality the result of avian predation (owls and accipiters).

The trio that I spotted at the Langenau pond included a juvenile that may have been the offspring of the female, but not necessarily that of the male. They migrate in groups of up to 45 birds, so the ones I saw may have been waiting to be able to fly again before joining others and heading south. Maybe they will find the trio I saw last week in the beaver swamp on Campground Road.