16-19" (41-48 cm.). A quiet, secretive duck, the Hooded Merganser is the smallest of the North American mergansers, about the same size as the Wood Duck. Hooded Mergansers are sexually dimorphic. Males are a striking combination of black, white, and reddish brown. The head, neck, and back are black, the rump is gray, and the tail is dark, grayish brown. The chest, breast, and belly are white, and there are two irregular black marks on the sides of the breast. The sides and flanks are tawny or reddish brown. The male’s most distinguishing physical characteristic is his crest, which when fully erect, reveals a stunning white patch bordered by black. When the crest is down, there is a white stripe extending backwards from the eye.
Females and immature males look alike. They are dark, grayish brown or blackish brown. The neck, chest, sides, and flanks are gray. The head is brown. The female’s crest is brown, tinged with cinnamon, and sometimes white at its tip. The crest of the immature male is similar to, but smaller than that of the female. Not all immature males have a crest.
Distribution and Breeding Habitat
Hooded Mergansers are the only merganser that lives (breeds and winters) exclusively in North America. In the past, they were found throughout the continent, including mountainous areas, wherever suitable habitats existed. Today, they are most common in the Great Lakes region and eastern Canada but are also found in the Pacific Northwest.
The habitat preferences of Hooded Mergansers are very similar to those of Wood Ducks. They like quiet, shallow, clear-water pools that have sandy or cobblestoned bottoms. They prefer ponds that are near or surrounded by deciduous woods: river bottomlands, small forest pools, millponds, swamps, and beaver ponds. Unlike Wood Ducks, Hooded Mergansers have a difficult time finding food in turbulent water, so calm, clear water is highest on their list of habitat requirements. Hooded Mergansers are not likely to nest on large lakes.
Information on the breeding ecology of Hooded Mergansers is scarce, despite its use of nest boxes. Overhunting and logging at the turn of the century depressed population numbers, but nest-box programs in states such as Missouri, Maine, Iowa, and Oregon have helped increase local populations. This species also seems to be particularly sensitive to pesticides and acid precipitation.
True to its scientific name, which translates to "hooded diver," Hooded Mergansers are experts at diving and foraging underwater. Their legs are situated farther back on the body than on dabbling ducks, such as Mallards. This makes mergansers awkward on land but expert divers and swimmers. They also have eyes well adapted to seeing underwater.
The diet of Hooded Mergansers is the most diverse of the mergansers. They dine on small fish, frogs, tadpoles, and invertebrates such as aquatic insects, snails and other mollusks, and small crustaceans. Hooded Mergansers may also eat seeds and aquatic plants.
Pair Formation and Territoriality
The information on territoriality and pair formation in this species is scarce. The timing and duration of pair formation is uncertain. Some birds return to the breeding ground together as mated pairs; others participate in courtship and mating rituals after arriving.
The date mergansers arrive on their breeding grounds and begin to breed depends on region and latitude. Hooded Mergansers are early migrants, and many individuals arrive just days after the ice breaks. Birds tend to arrive in February in Missouri, late March in the Great Lakes region, and mid-to-late April in British Columbia. Female Hooded Mergansers show strong philopatry, returning to the same breeding area, but not necessarily the same nest site, as in previous years. Nesting begins as early as February and early March in the lower latitudes and milder regions of the merganser’s range. Birds breeding at higher latitudes begin to nest from early April to early May.
Hooded Mergansers are monogamous, and they do not breed until their second year. Yearling males, nevertheless, may perform some courtship displays and form loose pair bonds.
Courtship groups consist of one to two females and several males. Males perform courtship rituals and displays, including crest-raising, head-shaking, head-throws, and head-pumping. Although normally silent, males make several vocalizations during some courtship displays. The most common is a rolling, frog-like call that is given immediately after a male performs a head-throw display. Males also may make short display flights. If anything, females perform only head-pumping displays.
Nest Building: Hooded Mergansers nest 10 to 20 ft (3 to 6 m) off the ground in hollow trees, hollow stumps, other natural cavities, and nest boxes. They prefer to nest near water but have been known to nest up to one mile from water. The female chooses the nest site. She does not add nesting material, instead forming a shallow cup from the material already present in the cavity. The cup is lined with down feathers plucked from the female’s belly, although some females may wait to lay their down after egg laying has begun (see Egg Laying).
Hooded Mergansers compete strongly for nest sites with other Hooded Mergansers and with other species such as Wood Ducks, Common Goldeneyes, and Common Mergansers. In fact, it is common to find the eggs of two species in one nest. Nest boxes placed at great distances from each other can help alleviate the problem of nest-site competition.
Nest boxes should be placed close to or over water. Hooded Mergansers nest in the same type of nest box used by Wood Ducks. One nest-box study investigating nest-site preferences of Hooded Mergansers and Wood Ducks found that proximity to water was the most important nest-site feature to Hooded Mergansers. They chose boxes that were closest to water. Wood Ducks, on the other hand, had no significant preference for boxes near water. Hooded Mergansers also prefer nest boxes that have been used in previous years and boxes that contain wood chips.
Egg Laying: Females lay one egg every other day, but some may lay the first three eggs of the clutch on successive days and the remaining eggs on alternate days. Eggs are smooth, glossy, white, and almost spherical. The average clutch size is 10 eggs, but clutches can contain as few as 5 and as many as 13. Clutch size in this species is correlated to the age of the female and time of the breeding season; in other words, clutches laid by older females and clutches laid earlier in the season tend to be larger.
The eggs are covered with a layer of down that is placed in the nest before egg-laying, during egg-laying, or after the clutch is completed. During this time, if the female is disturbed before she has finished lining her nest with down feathers or laying her clutch, she will abandon the nest.
Incubation: The incubation period is 32 to 33 days. Once the female begins to incubate, the male leaves the nest site and is gone for the remainder of the breeding season. If the female is disturbed toward the end of the incubation period or when she has ducklings, she will perform a "broken wing" distraction display.
Nestling Care: Hatching in Hooded Mergansers is highly synchronous; the eggs usually hatch within four hours of each other. The ducklings are precocial—covered in down, mobile, and able to feed themselves. They remain in the nest for 24 hours where they are brooded by the female. The female then leaves the cavity, calls to her young using a soft, guttural vocalization, and leads her brood to a nearby pond that is rich in invertebrates and small fish. The young are able to dive, but first dives are shallow and brief.
The female alone cares for the ducklings. After 70 days, the young can fly, and the female leaves them and prepares for fall migration.
Hooded Mergansers have one brood per season. Females rarely lay replacement clutches. If a nest fails early in incubation, when the male is usually still present, a female may renest. If the male has already left the breeding area when a nest fails, the female’s chance to breed for the year is gone.
Winter Movement and Dispersal
Hooded Mergansers migrate in late fall, and they migrate singly, in pairs, or in small flocks. They migrate short to intermediate distances. Most individuals in the northern part of the breeding range move to the southern and coastal regions of the continent, wintering mainly on freshwater. Some winter as far north as the ice permits. Birds breeding in mild climates may stay and reside throughout the winter. There is little information on dispersal in this species. Yearling females, although not old enough to breed, seem to return to the general area as their natal nest cavity but not to the same nest site. Older females, who have bred in previous years, show extreme site fidelity, often returning to the same nest box as the year before.