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Long-term Declines in Nest Success of Prairie Ducks

Results


Over all sites, nest success declined over time (F = 14.93; 1, 141 df; P = 0.0002; Fig. 2). Year alone, however, accounted for little variation (r2 = 0.10) in nest success. Estimated mean nest success over all study sites was 33% (95% CI 21-52) in 1935 (the earliest yr in the analysis), 21% (95% CI 17-28) in 1955 (the first yr of breeding-ground population surveys); 15% (95% CI 13-18) in 1970 (the beginning of a decade with particularly high populations), and 10% (95% CI 8-12) in 1992 (the most recent yr in the analysis).

Figure 2

Fig.2.   Decline in nest success of 5 upland-nesting species of ducks (pooled) at 67 study sites in Prairie Pothole Region during 1935-92 (n = 143). Predicted nest success (——) and confidence intervals (– – –) are shown for the regression.

In Canadian prairie-parkland sites alone, nest success also declined over time (F = 16.88; 1,73 df; P <0.0001). Analysis of residuals indicated that conserved soil moisture did not explain any additional variation in nest success after variation due to year was taken into account (F = 0.07; 1,72 df; P = 0.79).

The full ANCOVA model indicated that there was no interaction between species and region over time in the unpooled data set (F = 1.37; 9,212 df; P = 0.94). Subsequently, ANCOVA models to test for differences among species and between regions were conducted separately (Freund et al. 1986:202-203). Rates of decline in nest success did not differ among species (F = 1.82; 4,222 df; P = 0.13), but nest success did (F = 3.68; 4,226 df; P = 0.006). Northern pintails and mallards (which did not differ significantly from each other, P = 0.45) had consistently lower nest success (P = 0.004) than did northern shovelers, blue-winged teal, and gadwalls (which did not differ significantly from each other, P = 0.18; Fig. 3). Neither nest success (F = 1.20; 1,140 df; P = 0.27) nor its rate of decline (F = 0.95; 1,139 df; P = 0.33) differed between grassland (n = 85) and parkland (n = 58) regions.

Figure 3

Fig. 3.   Decline in nest success for each of 5 upland-nesting duck species at 49 study sites in the Prairie Pothole Region, 1935-92. Species that nest early (mallards [n = 81] and northern pintails [n = 40]) have lower intercepts than those that nest later (northern shovelers [n = 29], blue-winged teal [n = 59] and gadwallws [n = 23]).


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