The ducks are swimming in loose formation, the late afternoon sun sparkling in their wakes.

Riding low in the water, one of the birds dives and several others immediately follow. A moment later they surface, their progress upstream uninterrupted.

Deliberately, they swing a little closer to shore, then turn in unison, herding their underwater prey into the shallows on the opposite side of the river.

As if released by a starter's gun, the entire line of red-breasted mergansers (Mergus serrator) suddenly dives into the trapped school of fish. The birds abandon their orderly pursuit of a meal as the fish-in full panic mode-dart in every direction. The bejeweled water is roiling with the feeding frenzy. The dripping mergansers surface momentarily before diving and dining anew.

On the far shore, a snowy egret seems momentarily startled by the commotion. The confusion only lasts a moment. The mergansers have delivered a feast right to the snowy's golden feet. The brilliantly white bird with the black dagger bill stabs a fish, flips it into the air and swallows it whole.

The red-breasted mergansers seem oblivious to the lucky egret as they continue to snap up fish as quickly as they can dunk their heads and put their serrated thin bills into action.

This small tidal river is too salty for common or hooded mergansers. The red-breasted merganser, however, can tolerate the saline waters, and these birds are enjoying the bounty found here.

The red breast that gives the birds their name is not nearly as remarkable as the merganser's orange-red bill. The long, needle bill with teethlike serrations seems the antithesis of a traditional duck bill. Although it is most pronounced in the red-breasted merganser, the thin bill structure is similar across all three merganser species and is a useful field identifier. It is also well-suited for gripping slippery fish.

Male red-breasted mergansers have a white neck ring above a breast that features red, black and white feathers. The overall effect is a modestly red tinge to the male's breast. His head is dark green with a ragged crest extending from the back of the head. The male's wings are black and gray with white patches.

The female's head and crest are faintly reddish, as is her neck. She lacks the white neck ring and reddish breast of the male. Her body and wings are a nondescript brown-gray over her white belly.

Both sexes have orange-red feet.

Mergansers typically breed from the Canadian Maritimes across to Alaska. As soon as the female starts to incubate the eggs, the male leaves the nest. She will hatch the eggs and care for the young on her own.

Like sea ducks, red-breasted mergansers have a salt-expelling gland that allows them to fish and live in saline waters.

In the winter, these birds can be found in long ribbons hugging both the Atlantic and Pacific coasts from Washington state and Maine all the way down to South America. With its miles of estuarine systems, the Chesapeake is a common wintering location.

As I have just witnessed, red-breasted mergansers are diving ducks, completely submerging as they search for food. When they are in the fishing mode, they can compress their feathers tightly around their bodies, eliminating air and reducing their buoyancy. Consequently, they ride low in the water.

Gathering themselves up in the split second before they dive, they give their heads an almost imperceptive lift before they disappear underwater.

Red-breasted mergansers use their large webbed feet to propel themselves through the water. Because of their agility, these ducks can swim surprising distances underwater, surfacing many yards away from the point where they began their dive.

As the feast comes to a close, I am left thinking about the vivid example of cooperative behavior I've just witnessed. Cooperation and other complex social phenomena are common in the avian world.

A "selfish" bird wouldn't issue a warning cry when a predator approached; it would simply head for cover. But the alarm cries of many species are well-documented.

Similarly, the "sentinel" geese on the edges of a feeding flock are routine sights in suburban parks and golf courses across the country. These individuals give up their own feeding time to allow their group to feed in relative peace. They'll get their own uninterrupted feeding later.

This popular refuge is filled with people of all stripes. I wonder how many of our complex human interactions are mirrored in the avian world.

Who among us takes on the role of sentinel? Which of us makes cooperation a central part of our lives?

And perhaps there's a lesson here, too. In our hyper-competitive lives, generosity sometimes seems in short supply. Like the red-breasted mergansers, shouldn't we be a bit more willing to share in our bounty?