Common mergansers on the pond in the rain

The rain is gentle today, but still falling hard enough to blur the white shapes on the pond. At first I thought they were just the little ice burgs you see floating on the lakes this time of year, but I drive past here every morning and i knew this pond had been open for a while. I stop my car in the middle of the road because ducks are more important than whatever appointment I am late for, and grab the binoculars that are ever present on the passenger seat beside me, (and yes, if you ride shotgun in my car you will have to carry them in your lap.) Three common mergansers glide effortlessly across the water like little Irish step dancers, immobile bodies graciously moving across the surface while frantic feet churn beneath them.

The water droplets on my 10X 30's create a fun house mirror effect. Huge rusty heads with bulging foreheads and needle thin red bills with bright white undersides that glare into rainbows though the greasy film on my binoculars lenses. I watch for a moment, then the green head of the male dips sharply and dives, a practiced motion that turns him into a feathered fish for as long as he can hold his breath.

So..... why do we call these birds common?