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Factors Associated with Duck Nest Success
in the Prairie Pothole Region of Canada

Seasonal Variation in Nest Predation Rate

We examined seasonal effects on nest success by comparing daily rates of nest predation for each of the 5 common species among search periods. This rate is defined as number of nests unsuccessful because of predation, divided by total exposure days. We compared daily predation rates for mallard and northern pintail nests under observation in first, second, or third search periods. For the later-nesting gadwalls, blue-winged teals, and northern shovelers, we compared rates only for second and third periods because insufficient nests were found in the first period. We used only nests found in uplands and dry wetlands and combined all nests in these categories. We excluded nests located over water because our sample size was insufficient for separate analysis.

We used daily rate of nest predation as the response variable in our analysis. Explanatory variables were area-year, search period when nest was found, and the interaction between these 2 variables. To examine interaction with search period, it was necessary to exclude (Appendix D) area-years for individual species unless at least 1 nest was found during each search period. We were left with 21-29 area-years for each of the 5 common species. Observations were weighted by the number of exposure days. Chi-squared statistics for each effect were calculated from Type III sums of squares (Johnson 1990).

Detection of a significant interaction between area-year and search period for 4 of the 5 common species prompted us to examine more closely the relation between drought conditions and daily rate of nest predation. To do this we grouped area-years into 3 wetness intervals by means of a centroid clustering procedure (SAS Institute, Inc. 1989). Intervals were based on percentage of seasonal wetlands in each area-year that were wet in May and on departure in total precipitation from the long-term average for that area during April through June. The 3 intervals were dry (<43% of wetlands wet and precipitation <-4% of average), moderate (>70% of wetlands wet and precipitation <-11% of average), and wet (>46% of wetlands wet and precipitation >7% of average). Within each wetness category (dry, n = 11; moderate, n = 7; wet, n = 13), we used linear contrasts to examine interactions between area-year and search period by species, again with daily rate of nest predation as the response variable.

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