36 



Birds — Our Uving Resources 



Table. Estimated annual niniihers 

 (in thousands) and recent trends 

 (1984-93) of ducks based on the 

 survey areas monitored by breed- 

 ing and midwinter surveys. 



1.0 



Redhead. 



Canvasback 



0.2 



0.0 |.i |M| ,l| M | M^ , l |,, ( ,l | ,l|ll| MM I|ll | 



54 57 60 63 66 69 72 75 78 81 84 87 90 93 

 Year 



Fig. 6. Redliead and canvasback 

 breeding population estimates. 

 1955-93 



^Elders include common eider {Somalena mollissima). king eider (S 

 spectabilis), spectacled eider (S, fischeri). and Steller's eider {Polyslicta 

 stelleri] 



"Goldeneye include Barrow's goldeneye {Bucephala islandica) and com- 

 mon goldeneye (6 clangula) 



'Mergansers include hooded merganser {Laphodytes cucallalus). red- 

 breasted merganser {Mergus senata). and common merganser (U mer- 

 gansei) 



''Scoters include black scoter (Melanitta nigra), surf scoter (W. 

 perspcillata). and white-winged scoter (M fusca) 



some of these species are (difficult to itientify 

 during aerial surveys, or are encounteie(i rarely, 

 they are combinetJ with related species {see 

 Table). 



Collectively, breeding populations of mer- 

 gansers and their allies were 9% lower in 1993 

 compared to the 1955-92 average. Merganser, 

 oldsquaw (Ckmgitla hyemalis], eider, and scot- 

 er breeding populations in 1993 were all lower 

 than their 1955-92 averages (see Table for 

 species). The breeding population of goldeneye 

 in 1993 was similar to the 1955-92 average, 

 whereas the bufflehead (Biicephala albeola) 

 breeding population was higher than the long- 

 term average. During the last 10 years, breeding 

 populations of eiders, oldsquaw, and scoters 

 decreased, bufflehead increased, and goldeneye 

 and mergansers were stable (Table). Winter 

 population estimates during 1983-92 decreased 

 for oldsquaw, increased for bufflehead and mer- 

 gansers, and were stable for other species in the 

 sea duck tribe (Table). 



In the United States and Canada, wood 

 ducks are the only representative of the tribe 

 Cairinina and ruddy ducks iO.xyiira jaimiicen- 

 sis) are the only representative of the Oxyurini 



tribe. Wood ducks are hard to survey because 

 they inhabit forested wetlands where it is diffi- 

 cult to obtain reliable counts. Their current pop- 

 ulation, however, is greater than in the eariy 

 1900's (Bellrose 1980). Midwinter counts of 

 wood ducks during 1983-92 indicated a stable 

 population (Table). Ruddy duck breeding popu- 

 lations in 1993 were similar to the 1955-92 

 average. 



Factors Affecting Population 

 Status 



Duck population changes occur on breeding, 

 staging, and wintering habitats, with the 

 changes on breeding habitats having the great- 

 est effect on populations. Degradation and 

 destruction of wetlands over the last 200 years 

 have diminished duck populations; wetland 

 alteration and degradation continue. The rate of 

 wetland loss has been greatest in prime agricul- 

 tural areas such as the Prairie Pothole region 

 (Fig. 1). and lowest in northern boreal forests 

 and tundra. Thus, species such as dabbling 

 ducks that mostly nest in the severely altered 

 Prairie Potholes have been harmed more than 

 species like sea ducks and mergansers that nest 

 farther north (Bellrose 1980; Johnson and Grier 

 1988). 



Because most dabbling ducks need grassy 

 cover for nesting (Kaminski and Weller 1992), 

 conversion of native grasslands to agricultural 

 production, including pastures, has reduced 

 available nesting cover and contributed to a 

 reduced nesting success for dabblers. This con- 

 dition is especially true in the Prairie Pothole 

 region of the United States and Canada (Fig. 1 ). 

 In addition, highly variable precipitation in the 

 Prairie Potholes has changed the number of 

 wetlands available for nesting. For example, in 

 1979 there were 6.3 million wetlands in the sur- 

 veyed portion of the Prairie Pothole region, but 

 by the next spring, wetlands in the same area 

 had decreased 55% to 2.9 million. Two years 

 later they increased more than 100% to 4.2 mil- 

 lion. These annual changes can temporarily 

 mask the long-term declining trend in wetland 

 abundance across the Prairie Pothole region. 



The changing availability of wetland habi- 

 tats in the Prairie Potholes region causes sub- 

 stantial fluctuations in some duck populations. 

 During periods of high precipitation, larger wet- 

 land basins are full or overflowing, and shallow 

 wetlands are abundant. Species such as the 

 northern pintail, which tend to use shallow or 

 ephemeral wetlands for feeding, produce more 

 young when wetland numbers increase (Smith 

 1970; Hochbaum and Bossenmaier 1972). 

 Consequently, population numbers increase as 

 they did during the 1970"s. 



