Dabblers and Divers: Life On and Under Water
There are rough drafts in nature; there are, in creation, ready-made parodies; a bill which is not a bill, wings which are not wings, fins which are not fins, claws which are not claws, a mournful cry which inspires us with the desire to laugh, there is the duck.
– Victor Hugo
The Arctic Ocean is a long way from a bread-strewn pond in the park, but there are ducks there, 50 metres down, foraging in the near darkness for shellfish and crustaceans. The oldsquaw propels itself to these depths with webbed feet set far back on its body—a characteristic of the diving ducks. Occasionally, it supplements its paddling with a flap of its wings. Reaching the bottom is not easy, even for these strong swimmers. That’s because ducks rely on a layer of down and air, trapped beneath their flight feathers, to keep warm. In effect, they are swimming inside a bubble, and it’s far lighter than the blubber that insulates diving mammals. This natural life jacket is most buoyant in the first few metres of the duck’s descent, before water pressure compresses it. Not until the oldsquaw passes a depth of some 45 metres is it able to stay down without effort.
Once on the bottom, the oldsquaw tears mussels or other bivalves from the rocks with its blunt and powerful bill. It may crack the mussel’s shell or swallow it whole, underwater, allowing its gizzard to do the work. The gizzards of sea ducks are so strong that it’s possible to hear the grinding of shells inside the duck.
Oldsquaws are among the most specialized of the diving ducks, which is one of the two broad types of duck. While all ducks feed on and around water, diving ducks regularly forage by submerging their whole bodies. Divers include the goldeneyes, canvasback, scoters, eiders, and mergansers. The sea ducks eat mainly fish and shellfish. Some freshwater diving ducks fish, but many grub in the sediment for aquatic tubers.
The other kind of duck is the dabblers. Dabbling ducks feed by dipping their spatulate bills into the water or mud. They also forage in shallow water by “tipping up”—reaching to the pond bottom with their bills while their feet and tail feathers wave in the air. Mallards are dabbling ducks, as are the shovelers, teals, pintails, wigeons, gadwall, and the American black duck.
The ducks are not particularly mindful of these categories, however. Dabblers may dive and divers may dabble. To further complicate matters, biologists recognize nine duck tribes, all but two of which are represented in North America. The four dabbling tribes are the shelduck tribe, the perching duck tribe, the whistling duck tribe, and the dabbling tribe (not to be confused with the general category of dabbling duck).
The three diving duck tribes represented in North America are the seaduck, pochard, and stifftail tribes. Some of these tribe names are somewhat misleading. Seaducks such as the harlequin and the surf scoter nest in forests many miles inland and will dive in fresh water.
The divers share a number of adaptations to hunting below the surface. Their feet are set well back on their bodies so that they can push from the rear. Their wings are generally not as broad as those of the dabblers, making them less buoyant. Some diving ducks flap their wings underwater to assist propulsion with their feet.
There are costs to these adaptations. The legs of some divers are positioned so far back that they can barely walk, leaving them more vulnerable to predators when they must come ashore to nest. Smaller wings provide less lift than broad ones, which can make for long takeoffs. The red-breasted merganser cannot take flight without first taxiing over the water for several seconds to get up to speed. By comparison, the larger wings of a dabbling duck such as the mallard can have it airborne in a single beat.
All birds have air sacs connected directly to the lungs, but in diving ducks these sacs are considerably smaller, reducing the duck’s buoyancy. Some biologists speculate that on long dives the air in these sacs may be stirred and exchanged with air in the lungs by the muscular action of swimming, in effect giving the diving duck a larger lung capacity. When a duck’s face enters the water, its heart rate immediately drops by about two thirds—particularly if the duck is forced underwater or frightened into diving. On long dives, blood flow is diverted from the other organs to the heart and brain to conserve the oxygen supply.