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Conserved soil moisture correlates positively with duck population sizes (Boyd 1981), but we found no evidence that it was associated with nest success. Perhaps the mechanisms we proposed (effects on residual vegetation or alternative prey) do not affect nest success, or are only weakly linked to CSM. For example, residual spring vegetation may also be affected by snowpack and subnivean harvest by rodents (Higgins and Barker 1982).

Nest success and its rate of decline did not differ between parkland and grassland regions, for all 5 species, despite inherent differences in composition of the predator communities between these regions (Sargeant et al. 1993). This suggests a causal agent (or agents) that act(s) at a large scale and affects all species and areas similarly.

Although the rate of decline was not different among species, average nest success differed. From lowest to highest, the order of adjusted means of nest success was northern pintail, mallard, northern shoveler, blue-winged teal, and gadwall. Interestingly, this ranking correlates with nesting chronology. Nest initiation dates vary among locations and from year to year depending on weather conditions but, in general northern pintails and mallards are early nesters (beginning in early Apr), northern shovelers are intermediate, teal are later (peak clutch initiation in late May), and gadwalls have the latest peak clutch initiation (Jun) of all dabbling ducks (Bellrose 1980). A similar ranking of nest success among dabbling duck species was found by Klett et al. (1988).

Interspecific differences in nest site selection, as well as timing of nest initiation, may account for some differences in nest success among species. Northern pintails tend to nest in open areas more than do other dabbling ducks (Bellrose 1980), and because vegetation is generally sparse in early spring their nests may be more prone to predation due to lack of concealment. Pintails tend also to nest in stubble fields where nest losses to farming practices, especially spring plowing, may be relatively greater than for other dabbling ducks (Milonski 1958, Klett et al. 1988). In our review, however, few studies from which we gleaned data had been done in cropland because most species usually nest in untilled habitats of marginal agricultural value, so we suspect that spring plowing cannot be a general explanation for the pattern of interspecific variation in nest success that we observed. Mallard nests may also be vulnerable to spring plowing (Bellrose 1980, Sugden and Beyersbergen 1985), but mallards generally use dense cover. Gadwalls nest in dense vegetation (Hines and Mitchell 1983), in part because they nest later in the season when vegetation is more fully grown. Dense vegetation may deter movement of some mammalian predators, as Duebbert (1969) speculated, and avian predators may not detect well-concealed nests (Clark and Nudds 1991). Further, there is evidence that some predators feed on duck eggs early in the season and later switch to alternative prey (Crabtree and Wolfe 1988). Therefore, late nests may, in general, be less vulnerable to predation.

There are several inherent limitations to testing hypotheses with historical data, but we think these problems do not seriously diminish our attempt to address the critical question of whether nest success declined over time. If bias exists, we think it might occur principally in the direction of overestimating the decline. First, particularly in the oldest studies, sites may not have been randomly selected and, not truly representative, because nest searches may have been conducted (either intentionally or unintentionally) in the most productive habitats. Second, only studies that found high nest success may have been published earlier. Third, the scarcity of studies of nest success before the 1970s, and the high variability in nest success reported then, may have resulted in a Type I error (i.e., we may have detected a false decline in nest success), though various cleavages of the data at different times do not result in substantially different conclusions about the temporal variation in nest success. On the other hand, early estimates of nest success, relative to later areas, could underestimate productivity. Nest abandonments were counted as nest failures in older studies, whereas many recent studies did not use abandoned nests if the cause was suspected to be observer-induced. Also, although female success (Cowardin and Johnson 1979) is infrequently reported, it may be a more appropriate index of productivity in some cases. Female success would equal nest success if no renesting occurred. However, female success may be much higher when renesting does occur (Cowardin and Johnson 1979), in which case productivity could be higher than our results indicate, especially in latter years.