42 



Birds — Oiii Liviiif; Re 



Most oflhe traditional stopover habitats used 

 hy migrating eanvasbaci^s no longer provide 

 suitable feeding and resting opportunities (Kahl 

 1 99 1 ). 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 I980's. 

 However, reciird drought in 1988-89 and exten- 

 sive flooding in 1993 in the Upper Mississippi 

 River basin have caused major declines in habi- 

 tat 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 ameri- 

 caiui). These improvements are attributed to 

 regulation of water discharges into the Great 

 Lakes and perhaps the proliferation of zebra 

 mussels (Dreisseua polymorpha). 



In the Pacific Flyway, coastal habitats used 

 by migrating canvasbacks have not changed 

 greatly since the 1930'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 duiing the 1970"s or I980'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 abun- 

 dance 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 habi- 



tats, canvasbacks are exposed to high levels of 

 ein ironmental 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 win- 

 tering 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 manage- 

 ment of canvasbacks include banding or radio- 

 telemetry data sufficient to provide habitat 

 information and estimates of region-specific 

 rates of survival, band recovery, and recruit- 

 ment; survival rates of immature birds between 

 hatch and anival on wintering areas; and cross- 

 seasonal effects of winter nutrition and contam- 

 inant exposure on reproduction. 



References 



Accurso. L.M. 1992. Distribution and abundance of winter- 

 ing waterfowl on San Francisco Bay. 1988-1990. M.S. 

 thesis. Huniboldl State University, Acadia, CA. 252 pp. 



Anderson. M.G. 1989. Species closures — a case study of 

 wintering watertbwl on San Francisco Bay. 1988-1990. 

 M.S. thesis. Humbolt Stale University, Acadia. CA. 252 

 pp. 



Haramis. CM. 1991. Canvasback. Pages 17.1-17.10 m 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. Derieth. and W.A. Link. 1994. Flock 

 sizes and .sex ratios of canvasbacks in Chesapeake and 

 North Carolina. Journal of Wildlife Management 58: 12.^- 

 1.^0. 



Haramis. G.M., J.R. Goldsberry, D.G. McAuley, and E.L. 

 Derieth. 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 CM. Bunck. 1993. 

 Survival of hatching-year female canvasbacks wintering 

 on Chesapeake Bay. Journal of Wildlife Management 

 57:763-770. 



Haramis, CM.. 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. 



