40 



Birds — Our Li\ini; Risoiirces 



Canvasback 

 Ducks 



by 



William L. Holiiiiaii 



G. Michael Haramis 



Dennis G. Jorde 



Carl E. Korschgen 



John Takekawa 

 National Biological Service 



Ciiiivasbacks (Aythya valisineria) are unique 

 tn 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 

 iHvers; 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 prefened 

 foods, especially energy-rich subtenanean 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 fre- 

 quent hunting closures in all or some of the fly- 

 ways (Anderson 1989). canvasback numbers 

 declined from \'-)>f' to 1993 and remain below 

 the population goal (540.000) of the North 

 American Waterfowl Management Plan 

 (USFWSand 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 canvas- 

 backs during their first year are considered 

 important constraints on popidation growth. 



Population size = 620.540 - 2,873 (year) 

 /■2 = 011.P=0014 



55 



58 



64 



67 



73 

 Year 



76 



79 



82 



85 



91 93 



Figure. Estimated breeding popu- 

 lation of canvasbacks, 1955-93 

 (data from the U.S. Fish and 

 Wildlife Service, Office of 

 Migratory Bird Management). 



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 3()'7f would be needed to detect a sig- 

 nificant difference between years with 90% 

 confidence. 



The winter distribution of canvasbacks has 

 changed since the 1950"s, when most canvas- 

 backs (79%) were found wintering in the 

 Atlantic or Pacific tlyways. 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 availabili- 

 ty, but may also indicate differences in survival 

 and recruitment. 



Sex Ratios 



Canvasbacks have a highly skewed sex ratio 

 favoring males. Sex ratios of wintering canvas- 

 backs in Louisiana (1.6-1.8 males:female: 

 Woolington 1993) and San Francisco Bay (2.2 

 malesit'emale; 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. 

 1 994). Based on recent ( 1 987-92 ) MWI and sex 

 ratio data, we calculated that the continental sex 

 rafio 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 (Seine et al. 1992). 



Nichols and Haramis (1980) found no asso- 

 ciation between canvasback harvest regulations 

 and survival. However, an analysis of return 



