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Canvasback Ducks


by
William L. Hohman
National Biological Service
G. Michael Haramis
National Biological Service
Dennis G. Jorde
National Biological Service
Carl E. Korschgen
National Biological Service
John Takekawa
National Biological Service
Canvasbacks (Aythya valisineria) are unique to North America and are one of our most widely recognized waterfowl species. Unlike other ducks that nest and feed in uplands, diving ducks such as canvasbacks are totally dependent on aquatic habitats throughout their life cycle. Canvasbacks nest in prairie, parkland, subarctic, and Great Basin wetlands; stage during spring and fall on prairie marshes, northern lakes, and rivers; and winter in Atlantic, Pacific, and Gulf of Mexico bays, estuaries, and some inland lakes. They feed on plant and animal foods in wetland sediments. Availability of preferred foods, especially energy-rich subterranean plant parts, is probably the most important factor influencing geographic distribution and habitat use by canvasbacks.
In spite of management efforts that have included restrictive harvest regulations and frequent hunting closures in all or some of the flyways (Anderson 1989), canvasback numbers declined from 1955 to 1993 and remain below the population goal (540,000) of the North American Waterfowl Management Plan (USFWS and Canadian Wildlife Service 1994). Causes for this apparent decline are not well understood, but habitat loss and degradation, low rates of recruitment, a highly skewed sex ratio favoring males, and reduced survival of canvasbacks during their first year are considered important constraints on population growth.

Status and Trends
Canvasback population trends are monitored by means of annual Breeding Waterfowl and Habitat Surveys and Midwinter Waterfowl Inventories (MWI). Readers should refer to cited literature for additional information regarding methods.

Canvasback Numbers and Distribution

Between 1955 and 1993 population indices for canvasbacks fluctuated between 353,700 and 742,400 and averaged 534,000 ducks (Figure). The population showed a general rate of decline of 0.6% per year during the period; however, because population estimates are imprecise, annual differences are difficult to detect. For example, a population change of more than 30% would be needed to detect a significant difference between years with 90% confidence.

Figure. Estimated breeding population of canvasbacks, 1955-93 (data from the U.S. Fish and Wildlife Service, Office of Migratory Bird Management).
The winter distribution of canvasbacks has changed since the 1950's, when most canvasbacks (79%) were found wintering in the Atlantic or Pacific flyways. The proportion of the continental population wintering in the Central and Mississippi flyways increased from 21% in 1955-69 to 44% in 1987-92 as a result of declines in canvasback numbers at Chesapeake Bay and San Francisco Bay and increases in the Gulf of Mexico region. Only about 23,000 canvasbacks winter in Mexico, but numbers may be increasing (Office of Migratory Bird Management, unpublished data). Shifts in winter distribution probably reflect regional differences in habitat availability, but may also indicate differences in survival and recruitment.

Sex Ratios
Canvasbacks have a highly skewed sex ratio favoring males. Sex ratios of wintering canvasbacks in Louisiana (1.6-1.8 males:female; Woolington 1993) and San Francisco Bay (2.2 males:female; J. Takekawa, unpublished data) are lower than those observed in the Atlantic Flyway (2.9-3.2 males:female), but sex ratios apparently decreased in two mid-Atlantic states between 1981 and 1987 (Haramis et al. 1985, 1994). Based on recent (1987-92) MWI and sex ratio data, we calculated that the continental sex ratio for canvasbacks likely lies between 2.0 and 2.5 males:female.

Survival
Annual survival rates of female canvasbacks (56%-69%) are lower than those of males (70%-82%; Nichols and Haramis 1980). Survival rates also vary geographically (survival is greater in the Pacific Flyway than in the Atlantic; Nichols and Haramis 1980) and are positively related to body mass in early winter (Haramis et al. 1986). Survival of females in their first year probably is reduced relative to that of adults. Assuming that all surviving females return to their natal areas to breed, return rates for female canvasbacks breeding in southwestern Manitoba suggest that only 21% of hens survive their first year compared to 69% annual survival of older hens (Serie et al. 1992).
Nichols and Haramis (1980) found no association between canvasback harvest regulations and survival. However, an analysis of return rates for female canvasbacks in southwestern Manitoba indicated that survival of immatures was significantly related to harvest (M.G. Anderson, Ducks Unlimited-Canada, unpublished data). The canvasback season was closed in the Atlantic, Central, and Mississippi flyways during 1986-93, but about 8,000 birds were harvested annually in Canada and 10,000 in the Pacific Flyway. There is also a substantial illegal harvest of canvasbacks at some sites (Haramis et al. 1993; Korschgen et al. 1993; W.L. Hohman, unpublished data). However, the current level of hunting-related mortality is probably not limiting population growth. Rather, annual variation in recruitment and degradation and loss of breeding, migrational, and wintering habitats are more likely influencing population size.

Time-specific Survival Rates and Sources of Mortality
Survival rates for adults in spring and summer are unknown. In spite of a nationwide ban on the use of lead shot by waterfowl hunters, ingestion of spent lead shotgun pellets by waterfowl is common and likely will remain so for many years. More than 50% of spring-migrating canvasbacks captured at a major staging area on the Mississippi River had elevated blood lead levels (Havera et al. 1992). Lead-exposed birds have reduced body mass, fat, and protein (Hohman et al. 1990), so their subsequent survival and ability to reproduce and perform activities such as courtship, migration, or molt, may be compromised.
Nest success (i.e., embryonic survival) of canvasbacks is highly variable, especially for birds nesting on the prairies. For example, nest success in southwestern Manitoba in wet years was 54%-60%, but in dry years averaged only 17% (Serie et al. 1992). In spite of habitat loss and degradation, ranges in nest success observed in southwestern Manitoba were similar in 1961-72 (21%-62%; Stoudt 1982) and 1974-80 (17%-60%; Serie et al. 1992). Mammalian predation, especially by mink (Mustela vison) and raccoon (Procyon lotor), is an important factor affecting the nest success of prairie-nesting canvasbacks.
    Canvasback (Aythya valisineria).
Courtesy W.L. Hohman, NBS
Mortality of prefledged ducklings is high, especially during the first 10 days (C.E. Korschgen, unpublished data). In northwestern Minnesota, estimated survival rates for ducklings up to 10 days old ranged from near zero to 70%, but differed between sexes during the first 25 days of life (male > female; C.E. Korschgen, unpublished data). Predation and weather were the primary sources of duckling mortality. Survival of young between fledging and fall migration is unknown; however, production estimates calculated from harvest information (0.16-1.07 young:adult) suggest that recruitment rates for canvasbacks generally are low compared to other ducks.
Survival rates for fall-migrating canvasbacks have not been studied, but survival rates have been estimated at several major wintering sites. Adult and immature females had high winter survival at Chesapeake Bay (83%-100%; Haramis et al. 1993) and coastal Louisiana (> 95%; Hohman et al. 1993). Winter survival was lower at Catahoula Lake, Louisiana (57%-92%), where canvasbacks were not only shot illegally but where substantial numbers of birds were also exposed to lead (W.L. Hohman, unpublished data).

Habitat Trends
Historically, climate, grazing, and fire were major factors affecting habitats of prairie-nesting waterfowl. Since settlement, however, human activities, especially those related to agriculture, have had a major impact on the quantity and quality of breeding habitats. Nationwide, over 53% of original wetlands have been lost. Wetland losses in states where canvasbacks historically nested range from less than 1% (Alaska) to 89% (Iowa); however, deeper wetlands preferred by nesting canvasbacks probably have been drained to a lesser extent than shallower wetlands.
Northern lakes used by canvasbacks for molting and staging before fall migration probably have been least affected by human and natural perturbations. Nonetheless, disturbances related to commercial and recreational activities, nutrient enrichment of lakes resulting from sewage discharges and agricultural runoff, introductions of herbivorous fish, and alteration of lake levels for generation of hydroelectric power have reduced the suitability and use of some traditional staging areas in the southern boreal forest region.
Most of the traditional stopover habitats used by migrating canvasbacks no longer provide suitable feeding and resting opportunities (Kahl 1991). For example, of the more than 40 former migration stopover areas in the upper portion of the Mississippi Flyway, only Lake Christina in west-central Minnesota, two pools on the Upper Mississippi River, and two areas on the Great Lakes have peak populations of more than 5,000 canvasbacks (Korschgen 1989). Restoration efforts begun in 1987 at Lake Christina were successful in reestablishing submersed aquatic vegetation and canvasback use. Habitat on the Upper Mississippi River increased in extent from the mid-1960's to the late 1980's. However, record drought in 1988-89 and extensive flooding in 1993 in the Upper Mississippi River basin have caused major declines in habitat quality and abundance.
In the Great Lakes region, increased bird use of Lake St. Clair and Long Point on Lake Erie coincided with improved water quality and increased production of submersed aquatic plants, especially wildcelery (Vallisneria americana). These improvements are attributed to regulation of water discharges into the Great Lakes and perhaps the proliferation of zebra mussels (Dreissena polymorpha).
In the Pacific Flyway, coastal habitats used by migrating canvasbacks have not changed greatly since the 1950's, although development has increased in some areas (e.g., Puget Sound). Whereas use of some inland sites (e.g., Great Salt Lake, Utah; Malheur National Wildlife Refuge (NWR), Oregon; and Stillwater NWR, Nevada) declined during the 1970's or 1980's, canvasback use of Klamath Basin NWR, Oregon-California, and Pyramid Lake, Nevada, has increased.
Degradation of water quality in the Chesapeake Bay caused by nutrient enrichment, turbidity, and sedimentation reduced the abundance of aquatic plant and animal foods most important to canvasbacks in winter (Haramis 1991). Declining availability of plant foods caused canvasbacks to shift to mostly animal foods. Canvasback numbers declined in response to loss of aquatic plants in the Chesapeake Bay, but increased in North Carolina and Virginia where preferred plant foods were still abundant (Lovvorn 1989). Aquatic plants are now declining in the coastal areas of North Carolina and other wintering areas throughout the Atlantic Flyway. Unless the widespread decline of aquatic plant foods is reversed, the number of canvasbacks wintering in the Atlantic Flyway is not likely to increase.
San Francisco Bay is the most important wintering area for canvasbacks in the Pacific Flyway. Urban development there has greatly reduced available habitat. In remaining habitats, canvasbacks are exposed to high levels of environmental contaminants (Miles and Ohlendorf 1993). Canvasbacks make extensive use of salt evaporation ponds in northern San Francisco Bay (Accurso 1992). These ponds recently came under public ownership, but their management as tidal salt marshes will probably reduce their use by canvasbacks. Increasing numbers of canvasbacks have been observed recently on wetland easements and sewage lagoons in the northern San Joaquin Valley.
Increased numbers of canvasbacks are wintering in the Gulf of Mexico region, especially at Catahoula Lake, where, since 1985, peak numbers (up to 78,000 birds) have equaled or exceeded counts on traditional wintering areas such as Chesapeake Bay and San Francisco Bay. Birds appear to be attracted to Catahoula Lake because of its abundant plant foods and stable flooding regime (Woolington and Emfinger 1989). These birds are at risk of lead poisoning, however, because of the high density of spent lead shot contained in lake sediments.

Information Gaps
Information needs for improved management of canvasbacks include banding or radio-telemetry data sufficient to provide habitat information and estimates of region-specific rates of survival, band recovery, and recruitment; survival rates of immature birds between hatch and arrival on wintering areas; and cross-seasonal effects of winter nutrition and contaminant exposure on reproduction.
For further information:
William L. Hohman
National Biological Service
Southern Science Center
700 Cajundome Blvd.
Lafayette, LA 70506
References
Accurso, L.M. 1992. Distribution and abundance of wintering waterfowl on San Francisco Bay, 1988-1990. M.S. thesis, Humboldt State University, Acadia, CA. 252 pp.

Anderson, M.G. 1989. Species closures -- a case study of wintering waterfowl on San Francisco Bay, 1988-1990. M.S. thesis, Humbolt State University, Acadia, CA. 252 pp.

Haramis, G.M. 1991. Canvasback. Pages 17.1-17.10 in S.L. Funderburk, J.A. Mihursky, S.J. Jordan, and D. Riley, eds. Habitat requirements for Chesapeake Bay living resources. 2nd ed. Living Resources Subcommittee, Chesapeake Bay Program, Annapolis, MD.

Haramis, G.M., E.L. Derleth, and W.A. Link. 1994. Flock sizes and sex ratios of canvasbacks in Chesapeake and North Carolina. Journal of Wildlife Management 58:123-130.

Haramis, G.M., J.R. Goldsberry, D.G. McAuley, and E.L. Derleth. 1985. An aerial photographic census of Chesapeake and North Carolina canvasbacks. Journal of Wildlife Management 49:449-454.

Haramis, G.M., D.G. Jorde, and C.M. Bunck. 1993. Survival of hatching-year female canvasbacks wintering on Chesapeake Bay. Journal of Wildlife Management 57:763-770.

Haramis, G.M., J.D. Nichols, K.H. Pollock, and J.E. Hines. 1986. The relationship between body mass and survival of wintering canvasbacks. Auk 103:506-514.

Havera, S.P., R.M. Whitton, and R.T. Shealy. 1992. Blood lead and ingested shot in diving ducks during spring. Journal of Wildlife Management 56:539-545.

Hohman, W.L., R.D. Pritchert, J.L. Moore, and D.O. Schaeffer. 1993. Survival of female canvasbacks wintering in coastal Louisiana. Journal of Wildlife Management 57:758-762.

Hohman, W.L., R.D. Pritchert, R.M. Pace, D.W. Woolington, and R. Helm. 1990. Influence of ingested lead on body mass of wintering canvasbacks. Journal of Wildlife Management 54:211-215.

 Kahl, R. 1991. Restoration of canvasback migrational staging habitat in Wisconsin. Tech. Bull. 172. Department of Natural Resources, Madison, WI. 47 pp.

Korschgen, C.E. 1989. Riverine and deepwater habitats for diving ducks. Pages 157-189 in L.M. Smith, R.L. Pederson, and R.M. Kaminski, eds. Habitat management for migrating and wintering waterfowl in North America. Texas Tech University Press, Lubbock.

Korschgen, C.E., K. Kenow, J. Nissen, and J. Wetzel. 1993. Final report: canvasback hunting mortality and hunter education efforts on the Upper Mississippi River National Wildlife and Fish Refuge. U.S. Fish and Wildlife Service, LaCrosse, WI. 50 pp.

Lovvorn, J.R. 1989. Distributional responses of canvasbacks to weather and habitat change. Journal of Applied Ecology 26:113-130.

Miles, A.K., and H.M. Ohlendorf. 1993. Environmental contaminants in canvasbacks wintering on San Francisco Bay, California. California Fish and Game 79:28-38.

Nichols, J.D., and G.M. Haramis. 1980. Inferences regarding survival and recovery rates of winter-banded canvasbacks. Journal of Wildlife Management 4:164-173.

Serie, J.R., D.L. Trauger, and J.E. Austin. 1992. Influence of age and selected environmental factors on reproductive performance of canvasbacks. Journal of Wildlife Management 56:546-555.

Stoudt, J.H. 1982. Habitat use and productivity of canvasbacks in southwestern Manitoba, 1961-72. U.S. Fish and Wildlife Service Special Sci. Rep. Wildlife 248. 31 pp.

Woolington, D.W. 1993. Sex ratios of wintering canvasbacks in Louisiana. Journal of Wildlife Management 57:751-757.

Woolington, D.W., and J.W. Emfinger. 1989. Trends in wintering canvasback populations at Catahoula Lake, Louisiana. Proceedings of the Annual Conference of the Southeastern Association of Fish and Wildlife Agencies 43:396-403.

USFWS and Canadian Wildlife Service. 1994. North American waterfowl management plan 1994 update, expanding the commitment. U.S. Fish and Wildlife Service, Washington, DC. 40 pp.


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