Common Merganser ––our uncommon merganser

By Kate Persons |

At Salmon Lake late spawning sockeye salmon are still thrashing in the shallows of their spawning area. Gulls, ravens, diving ducks and a breathtaking pair of yellow-billed loons have been there too, feeding on sockeye carcasses, salmon eggs and the small fish and aquatic invertebrates that all come to eat at this autumn banquet.
Out of this wonderful stew of life, it was an octet of common mergansers that most captivated me. They were a family group – a female with seven large juveniles. To my knowledge, common mergansers seldom breed in this area and it was my first time seeing a brood here.
Mergansers are circumpolar, fish-eating sea ducks that dive energetically into our fresh and marine waters, snatching fish with slender red “saw-bills” that grip slippery prey.
We see two types of mergansers in this region – the frequently occurring red-breasted merganser that regularly breeds here, and the larger common merganser that is common here in name only. (You can read about red-breasted mergansers in the Birder’s Notebook in the August 19, 2021 edition of The Nome Nugget.)
Unlike their smaller, ground-nesting cousins, common mergansers typically nest in tree cavities. So, their normal breeding range is in northern forested areas such as interior and southern Alaska, where trees are large enough to provide hollow nest sites – hardly the scene at Salmon Lake! But they usually pick nest sites near clear, freshwater lakes or rivers with plentiful small fish, and Salmon Lake does fit that bill.
Although common mergansers generally don’t nest here, flocks are seen passing along the Norton Sound coast each spring and fall. And in late September and early October, flocks of up to 190 have been reported on eBird in the lower Kuzitrin delta.
These are cold-hardy ducks, and sometimes a few stay near Nome late into the fall, fishing in open holes and roosting on the ice edge with gulls and eiders. Once, we were surprised to find a common merganser at the mouth of the Nome River in December during the Christmas bird count.
The passage of these handsome, large mergansers made me wonder where they might be going to and coming from, so far from the forested habitat where they usually breed.
I asked Paul Lehman, an authority on bird distribution in the Bering Strait region, for insights about the common merganser’s comings and goings.
Lehman explained that many waterfowl species have what is called a “molt migration.” After breeding, primarily males who are not involved with raising the young, migrate to a distant location to molt and spend their flightless period in a food-rich, safe haven. As early as June some species migrate to areas north of their breeding range, bringing them to places like the Seward Peninsula, far from their breeding grounds. 
In 2005 and 2006, U.S. Geological Survey biologists tracked the movements of juvenile common mergansers that they satellite tagged on the Kenai Peninsula. Two of the birds survived through their second summer and their molt migrations were documented. The male flew to the Seward Peninsula and across the Bering Strait to Chukotka where it presumably molted, while the female remained close to its birthplace.
Mergansers don’t breed until they are at least two years old, and breeding male mergansers typically leave their nest site when incubation begins. Molt migrations of both young nonbreeders and post-breeding males, could largely account for the passage of common mergansers through the Bering Strait region.
Molt migrations reduce competition for limited food resources on the breeding grounds, maximizing food availability for growing youngsters and depleted females.
Although common mergansers usually nest in tree cavities, they do occasionally nest in rock crevices, in old buildings, or on the ground beneath dense vegetation, close to water. Apparently, Salmon Lake provided such a nesting situation this summer. And eBird indicates a few broods have been seen on the lake in the past.
In their customary breeding habitat, common mergansers have some interesting nesting behaviors.
In places where multiple common mergansers or other cavity nesting ducks breed, females may lay their eggs in the nests of these other ducks (egg dumping), who then incubate and rear the ducklings as their own.
The ducklings stay in their nest only a day or two before venturing to the exit and tumbling from tree nests, in some cases as far as 100 feet, to the forest floor below. The mother guides her brood to food-rich waters where the young feed themselves, occasionally toting a few young ducklings on her back.
Common mergansers mainly eat fish, supplemented with fish eggs, insects, mollusks, crustaceans and worms. Ducklings start out diving for aquatic invertebrates and begin fishing in about 12 days.
The family I watched at Salmon Lake was feeding in typical merganser fashion. They swam along the shoreline with their heads submerged, looking for prey. When one dove, the rest followed in quick succession, plunging in with slight upward and forward leaps.
While they can dive for up to two minutes, they usually submerge for less than 30 seconds. At Salmon Lake their dives amidst the spawning sockeye were brief, perhaps just long enough to grab loose salmon eggs.
Typical of diving birds, a common merganser’s legs are located far back on the body so their big feet can propel them efficiently when swimming and diving. This makes them awkward on land and they seldom walk.
They often rest and preen on gravel bars and shorelines, quickly launching into the water or air if alarmed, usually scampering and splashing over the water before liftoff.
In Alaska common mergansers winter in the Aleutians, at Kodiak Island and along the coast of southern Alaska.
Indications are that common merganser numbers in North America are stable.
After a seemingly successful breeding season, I will be curious to see if they return to nest at Salmon Lake next spring.