Northern Prairie Wildlife Research Center
In 1992 and 1993 we searched 9,567 acres (3,872 ha) in 137 CRP fields and 5,745 acres (2,325 ha) in 95 planted cover fields on 52 study sites (Figure 1). We found 1,197 duck nests in CRP cover and 624 duck nests in planted cover, of which 1,121 and 578, respectively, could be used for estimating DSR. Principal species in CRP cover were blue-winged teal (32.1 percent), gadwall (A. strepera) (28.3 percent), mallard (21.6 percent), northern shoveler (A. clypeata) (8.6 percent) and northern pintail (8.4 percent). The species composition in planted cover was blue-winged teal (51.4 percent), gadwall (22.9 percent), mallard (12.2 percent), northern shoveler (10.3 percent) and northern pintail (3.0 percent). Sufficient information to develop models of DSR was obtained for mallard, gadwall and blue-winged teal.
Mallard. The four regression lines for DSR of mallard did not differ (F3,66, P = 0.09, P = 0.966), indicating no treatment or year effects. All data were combined into a single model that indicated DSR increased from east to west (F1,66 = 8.34, P = 0.005).
Gadwall. Daily survival rates of gadwall nests increased from east to west (F1,82 = 4.49, P = 0.037) and the rate of increase did not depend on treatment or year (F3,79 = 0.50, P = 0.681). The regression line for CRP in 1992, however, was lower (P<0.05) than the other three lines. The other three did not differ from each other (P>0.05).
Blue-winged teal. Daily survival rates of blue-winged teal nests varied from east to west, but not linearly or consistently from year to year (P<0.05). The shapes of the curves for CRP and for planted cover were the same within each year, but differed between years. Tests indicated that DSR was higher in CRP than in planted cover in 1992 but the two did not differ in 1993.
Our overall estimates of nest success for CRP and planted cover in North Dakota were higher than estimates of nest success in planted cover reported by Klett et al. (1988) for 1980-84 (Table 2).
|Table 2. Comparison of estimated nest success in Conservation Reserve Program cover and planted cover in 1992-93 with estimates for planted cover in 1980-84a for mallard, gadwall, and blue-winged teal.|
|Habitat||Nest success (percentage)|
| a From
Klett et al. 1988.
b Habitat not available in 1980-84.
Prior to the CRP, all eight of the CRP-influenced species had negative estimates of trends, but post-CRP, four of the species had positive trend estimates (Table 1). In contrast, four of the twelve CRP-neutral species had negative point estimates of trends pre-CRP, but seven had negative trends post-CRP. Evaluation of differences in trend (trendPost-CRP – trendPre-CRP) indicates that the CRP-influenced species were more likely to be increasing during the later period (mean differences: 4.72 [CRP-influenced], -3.19 [CRP-neutral], t = 1.73, df = 18, P = 0.052).