Wild Side: The common merganser

If it’s so common, why don’t we see it more often?

Courtesy U.S. Fish and Wildlife Service.

Work, medical woes, and off-Island travel have curtailed my birding in recent months, making most of my enjoyment of birds vicarious. I was happy, therefore, to see photos of one of my favorite birds, the common merganser, posted recently on a Vineyard birding Facebook page by the indefatigable birder and photographer Jeffrey Bernier.

Not exactly rare on the Vineyard, the common merganser occurs here regularly, but in very small numbers. It’s fair to call the species annual here, but the few individuals that turn up often don’t linger, and they tend to prefer a few not very accessible locations. So it’s easy for even an active birder to miss this species entirely on the Vineyard in a given year.

Squibnocket Pond, where Jeff Bernier found his recent one, is by far the favorite Island refuge of the common merganser. James Pond, adjacent to Lambert’s Cove Beach, is another location that this bird seems to favor (rather surprisingly, if you ask me). Great ponds, especially the coves, may also host this species, which tends to prefer large bodies of water in winter. Island records reflecting northbound migrants in spring are rare (I can’t recall any); for us, this is a bird of late autumn and winter. Even then, it takes persistence, luck, or both to find this duck on the Island.

Oddly, this state of affairs appears to be a recent development. “Vineyard Birds II,” that excellent compendium of Island bird records assembled by Soo Whiting and Barbara Pesch, points to lofty Christmas Bird Count numbers in years past, culminating in a remarkable tally of 236 in 1987. But these days, more than a handful is unheard of in a Vineyard Christmas Count, and some years the species is missed entirely.

I’m at a loss to explain the change. The Massachusetts Breeding Bird Atlas, compiled by hundreds of volunteers under the auspices of Massachusetts Audubon, shows that common mergansers have exploded as a breeding bird in the Bay State over recent decades. Virtually all of this increase was noted in western Massachusetts, but still, shouldn’t the Vineyard get some of that bounty?

More broadly, studies differ somewhat on the population trend for this large duck. The USGS Breeding Bird Survey suggests a slow, steady decline; but the Sea Duck Joint Venture project, relying on aerial surveys of and Christmas Bird Count data, estimates a fairly stable population of between 600,000 and 1,000,000 birds in North America. Heavily reliant on fish, common mergansers are vulnerable to pesticide and heavy-metal poisoning. On the other hand, the regrowth and maturation of forest in portions of this bird’s range create new opportunities for nesting.

It’s possible, I suppose, that past Vineyard Christmas Count tallies of common merganser were inflated by mistaken identifications of the similar red-breasted merganser. It’s an easy mistake to make, but given the skill level that prevails in the Island birding community, this is a bit hard to believe. Perhaps the ecology of our ponds has changed, rendering them less attractive to mergansers through loss of potential prey.

A duck with circumpolar distribution, the common merganser breeds in a broad swath all the way across North America, north to the limits of large trees. (Like other mergansers, these birds typically nest in cavities in trees, and since a common merganser may exceed three and a half pounds and two feet in length, you aren’t going to find them nesting in stunted vegetation.) While the population in North America is generally given status as a subspecies, our birds are virtually indistinguishable in the field from those in Europe and Asia.

Adult male common mergansers are elegant birds with largely white bodies and green heads. Females and immature males are far duller, mostly grayish with rusty brown heads. It is these plumages that are most likely to be confused with red-breasted merganser. A sharp demarcation of the reddish head from the pale neck, a heavier bill, and a distinct white spot on the chin are marks that help distinguish common mergansers from their relatives.

Common mergansers are also much less likely than red-breasted mergansers to occur on salt water. The wintering distribution of the latter species is much more coastal than that of the former, and a merganser on an open oceanic shoreline is unlikely to be a common. Red-breasted mergansers, unfortunately, are perfectly happy using fresh water, so you can’t use location to rule out that much more numerous species.

Fast and direct in flight, and incredibly deft in the water, neither merganser is worth much on dry land. Like loons, their legs are mounted far back on their bodies — good for swimming but not for walking. Mergansers feed exclusively in the water, diving for fish, other small aquatic vertebrates, and invertebrates such as worms and insect larvae.

I make a special trip off-Island most springs to see common mergansers in the Sudbury River Valley of eastern Massachusetts. Wintering northward to the limit of open water, these ducks pass through mainland Massachusetts reliably as the ponds and rivers thaw in March, even late February. The effort reflects how much I admire this large and beautiful duck. Their puzzling scarcity on Martha’s Vineyard is an irksome downside to being a birder on our otherwise congenial sandbar.