Long-term Declines in Nest Success of Prairie Ducks
Introduction
Declines of some duck populations in the Prairie Pothole Region (U.S. Fish and Wildl. Serv. and Can. Wildl. Serv. 1986, Anonymous 1989, Dickson 1989, Caithamer et al. 1992, Bethke and Nudds 1995) have led to speculation about ecological causes and management remedies. Hypothesized causes for the declines include decreased natality because of habitat alteration, drought, farming practices, and depredation of nests, and/or increased mortality from overhunting, environmental contaminants, disease, and predation. Predation has been identified as a principal agent of nest loss (Sargeant and Raveling 1992), and it is thought that nest success has declined because of increased losses to predators caused, ultimately, by intensive agriculture and widespread habitat change (e.g., Boyd 1985, Cowardin et al. 1985, Klett et al. 1988, Johnson et al. 1989).
Although the perception is widespread that nest success in the Prairie Pothole Region is lower now than before, the evidence is drawn from comparisons across different time scales and locales. Hammond and Forward (1956:246), for example, estimated that apparent nest success (the ratio of successful nests to no. of nests found) at a site in North Dakota decreased from 70-80% to 20-30% between the periods 1937-38 and 1947-51. From several studies in the Canadian and American prairies, Miller (1971) concluded that nest success had declined from 63% (apparent success) in the 1930s to 29% in the 1950s. Nelson and Duebbert (1973) suggested that nest success had decreased from 60-80% (apparent success) in the 1930s, to 30-40% in the 1950s, and speculated that a considerable decline had continued into the 1970s. Klett et al. (1988) however, compared nest success (Mayfield method) from 3 states in the Prairie Pothole Region between 1966 and 1984 and found little evidence of decline. Thus, it is still not clear whether nest success actually has declined since the 1930s and, if it has, to what extent.
A large number of nesting studies were conducted during the 1980s (Greenwood 1986, Johnson et al. 1988a, Greenwood et al. 1990, Clark et al. 1991) which was one of the driest decades since the 1930s. Long-term variation in nest success is potentially affected by variation in soil moisture conditions in at least 2 ways. First, soil moisture can affect the establishment of vegetation cover, therefore it may affect residual vegetation the following spring when ducks begin to nest. Reduced nest concealment may lead to lower nest success, at least in certain predator communities (Clark and Nudds 1991). Second, nest success of ducks may be positively correlated with small mammal abundance (Byers 1974, Weller 1979) or insects (Crabtree and Wolfe 1988), which may be alternative prey for nest predators. When dry conditions result in lower productivity of primary producers, alternative prey may become scarce and predators may consume a higher proportion of duck eggs (Johnson et al. 1989).
Further, the composition of the nest-predator community varies across the Prairie Pothole Region (Sargeant et al. 1993) and has changed over time (Johnson and Sargeant 1977). Because the effect of nest concealment on nest success differs with composition of the community of nest predators (e.g., avian vs. mammalian; Clark and Nudds 1991), we tested the extent to which temporal and spatial variation in nest success might differ between grassland and aspen parkland.
Using data compiled from published and unpublished studies (1935-92) of upland-nesting ducks in the Prairie Pothole Region, we tested whether nest success (1) declined over time, (2) was related to variation in soil moisture, (3) differed among species, or (4) differed between ecogeographic regions.
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