Canvasback (aythya valisineria)
Canvasback pair. Photo by Dick Pospahala.
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Midmorning on a brisk December day, a
waterman pauses from culling his catch as a shadow moves overhead. Looking
up, he sights a large, wedge-shaped mass of birds passing almost silently
above him; their oscillating wings flickering white in the filtered sunlight.
He smiles in appreciation of the sight before returning to his work; for
continuing an ancient ritual, the majestic canvasback is return ing to
spend another winter on the Chesapeake Bay.
At first glance, the elegant canvasback
resembles the redhead duck (Aythya americana), but can be easily distinguished
by its wedge-shaped head and bill profile. Males are white-bodied with
a reddish-brown head, black breast and tail, and red eyes. Females are
brightly plumaged, having a brownish-tan head and grey-brown body. In
the Chesapeake Bay, canvasbacks form large flocks of conspicuously white
birds in estuarine Bay habitats.
Life History
Canvasbacks are the largest of a group of widely distributed waterfowl
known as pochards, or diving ducks. Large webbed feet on legs located
toward the rear of their bodies make them agile underwater swimmers; however,
the location of the legs makes land travel difficult, therefore, they
are rarely found far from the water's edge. Canvasbacks use their long,
sloping bill to forage in bottom sedi ments for subterranean plant and
animal foods. Rootstalks, tubers and stems of submerged
aquatic vegetation (SAV), as well as bottom dwelling animals, such
as aquatic insects and small crustaceans, are some of the canvasback's
preferred foods. In Chesapeake Bay, the Baltic clam is a critical food
source for wintering populations of canvasbacks.
The largest proportion of canvasbacks nest on the North American prairies,
from Minnesota and the Dakotas through Manitoba, Saskatchewan and Alberta.
By mid-May, the females build a nest of cattail, bulrush, or whitetop
grass and lay a clutch of 8-10 greenish eggs. Once the eggs are laid,
the males move to larger lakes to molt, leaving the females to raise the
young alone. The ducklings hatch after approximately 25 days of incubation
and begin to forage in the wetland for small invertebrates and plants.
Poor weather, pesticide poisoning, and predation by mink and raccoons
can all have a detrimental impact on the eggs and ducklings. As winter
approaches, lakes and ponds freeze, harsh weather across the prairies
limits food availability, and canvasbacks migrate to warmer climates during
winter. During migration and winter, canvasback flocks aggregate in Chesapeake
Bay, San Francisco Bay, the Mississippi Delta region and adjacent Gulf
coast, and interior Mexico.
Historically on the Chesapeake, the Susquehanna Flats--the
large shoal area at the head of the Bay- -attracted tens of thousands
of canvasbacks because of the lush beds of their favorite food, a species
of SAV known as wild celery (Vallisneria americana). In fact, the Canvasback's
species-specific name, valisineria, was derived from the scientific name
for wild celery. Although the Bay is still important to canvasbacks, the
decline of SAV has forced the ducks to winter on other coastal brackish
waters where food is more abundant.
Population Decline
In the 1950s, the 250,000 canvasbacks, or "cans," that wintered on the
Chesapeake represented one half of the entire North American winter population.
Today, the Chesapeake harbors only about 50,000 cans—about one tenth
of the estimated North American winter population. Canvasback numbers
have substantially declined for several reasons. In the 19th century,
their seemingly endless abundance and excellent eating quality made the
canvasback a good winter food source and a favored dish in many restaurants
along the East coast. Their aggregated flocks or "rafts" in open water
made canvasbacks vulnerable to overharvest. By the end of the 19th century,
commercial hunters were using large-bore shotguns and batteries of cannon
and "punt" guns to assault rafts of canvas backs, killing dozens of birds
with one shot. The birds were then shipped in canvas bags by boxcar to
markets from Baltimore to Boston. The name "canvasback" may have originated
from the early method of shipping the ducks to market in canvas bags labeled
"canvasback" to indicate return of the bags for reuse, although another
explanation comes from the finely barred white plumage on the body of
the duck which resembles the texture of canvas fabric. Concern grew over
this unregulated harvest until commercial hunting ended with passage of
the Migratory Bird Treaty Act of 1918, which empowered the federal government
to set seasons and bag limits on the hunting of migratory game birds.
Of the many factors responsible for declining canvasback populations,
habitat degradation (wintering, migratory and summer nesting grounds)
has had the largest impact. The decline of water quality in the Chesapeake
has greatly reduced available food for canvasbacks as well as many other
species of waterfowl. Increased sedimentation from erosion has caused
a dramatic decline in SAV by reducing light penetration. Other problems
canvasbacks may encounter on the Bay include toxins entering their food
supply, disturbance from shoreline development and recreational activities,
oil spills, disease, and illegal hunting of females due to hunters misidentifying
canvasbacks as female mallards. Restoring the Canvasback
Like all migratory species, canvasbacks need dependable,
high-quality breeding, wintering and migration habitats. The prairie wetlands
(or potholes) of North America are vital to the survival of many species
of waterfowl, including canvasbacks. Many prairie potholes have been filled,
drained, or otherwise detrimentally impacted by agricultural practices.
The most important factor affecting the restoration of
canvasbacks and other species is improving water quality to enable regeneration
of SAV. Other beneficial actions include educating hunters, establishing
open water sanctuaries, and continuing research regarding habitat use
and food resources. These actions, combined with the protection of nesting
areas in North American prairies, are vital to the survival of these magnificent
waterfowl. If our restoration efforts are a success, then someday great
clouds of canvasbacks may again fill Chesapeake Bay.
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