Our Liviiif; Resources — Birds 



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rates lor lenialc canvasbacks in southwestern 

 Manitoba indicated that sufvival of immatures 

 was signilicantiy related to harvest (M.G. 

 Anderson. Ducks Unhmited-Canada. inipub- 

 hshed data). The canvasback season was closed 

 in the Atlantic. Central, and Mississippi flyways 

 during 1 986-03. but about 8.000 birds were har- 

 vested annualls in Canada and 10. 000 in the 

 Pacific Flyway. There is also a substantial ille- 

 gal harvest of canvasbacks at some sites 

 maramis et al. 1993; Korschgen et al. 1993; 

 W.L. Hohnian. unpublished data).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 inlluenc- 

 ing population size. 



Time-specific Survival Rates and Sources of 

 Mortality 



Survival rates for adults in spring and sum- 

 mer are unknown. In spite of a nationwide ban 

 on the use of lead shot by waterfowl hunters, 

 ingestion of spent lead shotgun pellets by water- 

 fowl is common and likely will remain so for 

 many years. More than 50% of spring-migrat- 

 ing 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 subse- 

 quent survival and ability to reproduce and per- 

 form activities such as courtship, migration, or 

 molt, may be compromised. 



Nest success (i.e., embryonic survival) ot 

 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 simi- 

 lar 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. 



Moitality of prefledged ducklings is high, 

 especially during the first 10 days (C.E. 

 Korschgen. unpublished data). In northwestern 

 Minnesota, estimated survival rates for duck- 

 lings 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 recruit- 

 ment 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-nest- 

 ing 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 histoiically nested range from less 

 than V7c (Alaska) to 89% (Iowa); however, 

 deeper wetlands preferred by nesting canvas- 

 backs probably have been drained to a lesser 

 extent than shallower wetlands. 



Northern lakes used by canvasbacks for 

 molting and staging before fall migration prob- 

 ably have been least affected by human and nat- 

 ural perturbations. Nonetheless, disturbances 

 related to commercial and recreational activi- 

 ties, 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. 



Canvasback {AMhya vulismeria). 



