Dabblers
vs. Divers
Nothing
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.
Dabblers, Divers, and Ducks That
Dive
DABBLERS
(Anas)
|
DIVERS AND STIFF-TAILED
DIVERS
(Aythya; Oxyura)
|
DUCKS
THAT DIVE
(But that are not usually
referred to as "Divers"; often
called "Sea Ducks")
|
Green-winged
Teal
|
Canvasback
|
American Black
Duck
|
Mottled
Duck
|
Redhead
|
Eiders
(Somateria, Polysticta)
|
Mallard
|
Ring-necked
Duck
|
Scoters
(Melanitta)
|
Northern
Pintail
|
Greater
Scaup
|
Harlequin Duck
(Histrionicus)
|
Blue-winged
Teal
|
Lesser
Scaup
|
Oldsquaw
(Clangula)
|
Cinnamon
Teal
|
Ruddy
Duck
|
Goldeneyes
(Bucephala)
|
Northern
Shoveler
|
Masked
Duck
|
Bufflehead
(Bucephala)
|
Gadwall
|
American
Wigeon
|
Mergansers (
Mergus, Lophodytes)
|
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.
SEE: Bird
Communities and Competition;
MacArthur's
Warblers;
Breeding
Season.
Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.
|