Bird of the Month by Russ Chappell
Common mergansers are marine diving ducks frequenting rivers and lakes in North America.
They have large, long bodies, thin, pointed wings, and straight, narrow bills. They are 23-28 inches long with 31-38-inch wingspans and weigh between two and 4.6 pounds. The males are slightly larger, with crests of longer head feathers normally lying smoothly behind the head, while females have shaggy crests on the backs of their heads.
Breeding males display white bodies with a salmon-pink tinge, black heads with an iridescent green gloss, grey rumps and tails, and primarily wings with white on the inner half and black on the outer. Non-breeding males and females are primarily grey, with reddish-brown heads, white chins, and white secondary wing feathers. Juveniles are similar to adult females, but display a short, black-edged, white stripe between their eyes and bill. Bills and legs are red to brownish-red, brighter on adult males, paler on juveniles.
Like other mergansers, their bills have serrated edges for better gripping of prey such as molluscs, crustaceans, worms, larvae and amphibians, occasionally small mammals and birds. Common mergansers float, loaf, fish, and often sleep on open water. They forage in groups along the shoreline seeking prey, and when spotted they dive, often as a group, staying submerged for up to two minutes.
They like nesting in tree cavities, and so prefer mature forests, although nesting boxes may be used when available. The female builds the nest from accumulated materials in the bottom of the cavity, and may add grasses, and after the eggs are laid the nest is lined with down from her breast. There is one brood per season, with eight or twelve white to yellowish eggs, and immediately after hatching the young are transported in the mother’s mouth to nearby waterway to feed on invertebrates and small fish. Common mergansers also form nurseries, often tasking a single female with overseeing dozens of ducklings.
Populations in North America declined by over 2% per year between 1966 and 2014, but they are not on the 2014 State of the Birds Watch List.
The Prescott Audubon Society is an official chapter of the National Audubon Society. Check it out online at PrescottAudubon.org.