Dabblers vs. Divers
Preening, pencil drawing of a Ruddy Duck by Terry Miller
is more fundamental to animals than eating. Consequently, it
is possible to organize a view of animal communities on the
basis of who eats what, where, and when, and who prevents
whom from eating. Many ecologists think that closely related
species tend to evolve strategies that minimize how much
they compete for food. Such "resource partitioning" has been
the subject of numerous investigations into the foraging
behavior of birds.
Not all species of waterfowl are highly competitive for food during breeding. Some, including geese and eiders that nest in the Far North, have minimal foraging requirements when on their breeding grounds because they build up sufficient stores of fat during the winter or when visiting their staging areas during migration.
Other migratory species, however, such as diving
and dabbling ducks that breed in the pothole region of the
northern prairies, wait until they arrive at their breeding
grounds to meet the bulk of their dietary demands and may
often compete for food. The shallow potholes (small ponds)
they use during breeding formed during alternating periods
of glaciation and semiarid conditions. The water in the
pools continually fluctuates in depth and salinity, and
consequently in the size of the resident communities of
plants and animals that can be supported. These pools have
provided ideal conditions for studying the behavior of these
ducks in detail.
Once assembled on the prairies in the spring, divers and dabblers appear to divide their habitat in a manner analogous to the way in which some warbler species divide trees in which they forage. This partitioning presumably has evolved to protect access to their preferred food items, which for breeding females consist of protein-rich aquatic insects, other invertebrates, and aquatic vegetation. During the period between their arrival and the laying of their eggs, females are essentially feathered eating machines, foraging from 50 to 70 percent of the day.
Diving ducks, or "divers," are ducks that propel themselves underwater with large feet attached to short legs situated far back on the body. "Dabblers," in contrast, have smaller feet and their legs are situated farther forward. While a few dabblers may occasionally dive to feed or to escape predators, typically they skim food from the surface or feed in the shallows by tipping forward to submerge their heads and necks. The table below lists the North American ducks generally included in the groups dabblers and divers. We have also listed a substantial group of species that dive after their food, but often are not meant when one refers to divers. Note that many of the ducks that dive also dabble. Although the Wood Duck (not listed) dabbles and shares with the dabblers the ability to take-off vertically, it is not ordinarily included in the dabblers.
In one study, wildlife biologists analyzed the patterns of foraging in three diving ducks whose ranges converge in the prairie pothole region. Presumably, in the past few thousand years, Redheads moved there from the southwest, Canvasbacks moved in from the east, and the range of Lesser Scaups expanded southward. These species may coexist within a given pool, but each occupies a particular area of water. Canvasbacks have the most specialized diet and usually take food from the bottom in open water near the center of the pool; females prefer immature aquatic insects and snails, often selected from relatively scarce species. Redheads have less specialized bills and presumably a more generalized diet of aquatic invertebrates and aquatic vegetation. They usually forage in open areas of shallower water near the edge of the pools and eat the most abundant food items; their diet changes in accordance with the relative abundance of food available. The foraging activity of the Lesser Scaups is dictated by their bills, which are specialized for straining small crustaceans (amphipods) from the water. The scaups usually seek food in the same central area as Canvasbacks, but remain in the open water column above the bottom.
When dabblers first arrive on their breeding grounds they eat seeds and waste grain exposed by snowmelt. As the season progresses and the emergent aquatic vegetation grows, their mainstay of surface-associated invertebrates increases in availability. But surface-feeding dabblers are not restricted to shallow water. Gadwalls, for example, feed as far from shore as Lesser Scaup and Redheads. Gadwalls, however, avoid direct competition with these two divers (and other dabblers) during the breeding season by delaying the onset of nesting.
The segregation of divers and dabblers within a pothole is not determined simply by the distribution of preferred foods, important as that is. It is also influenced by the access to takeoff and landing areas in the pool. Dabblers have large wings relative to body weight and fly slowly, which enables them to drop down onto small areas with precision. Divers, on the other hand, have small wings relative to body weight and fly faster, but must remain in open water with sufficient runway space because they lack the ability to land on a dime and must run along the water surface to become airborne.
Of course, such patterns of habitat division among divers and dabblers is not limited to species breeding in the prairie pothole region. The division of foraging habitats by these waterfowl is relatively consistent in different geographic regions although there is some variation according to the mix of species.
Many questions concerning competition and resource partitioning remain unresolved, but evidence from studies on divers and dabblers seems to support one tenet of ecological theory, that space is easier to divide than food.
Bird Communities and Competition;
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.