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Factors Limiting Mallard Brood Survival
in Prairie Pothole Landscapes

Results


Across study areas and years, seasonal ponds accounted for 91% of the variation in total pond numbers during May (F1,26 = 239.0, P < 0.0001), 97% during June (F1,30 = 831.5, P < 0.0001), and 94% during July (F1,29 = 446.6, P < 0.0001). All ducklings in 16 of 56 broods died during 1,250 exposure days (Table 1). We did not detect effects of TMIN, BA, PERNCOVER, age of brood female, brood size, or any interactions on survival (P > 0.05). Further, we did not detect an effect of radiotransmitters on brood survival (P = 0.68). Our final fitted model of brood survival contained main effects of WETSEAS (Wald χ²1 = 13.16, P = 0.0003), HATCHDATE (Wald χ²1 = 7.40, P = 0.007), and RAIN (Wald χ²1 = 5.79, P = 0.02). Total brood loss during the first 30 days of exposure was 11.2 (95% CI = 3.0-41.4) times more likely for broods hatched on areas with <17% WETSEAS than those on areas with >59% WETSEAS (Fig. 1A). Total brood loss was 5.2 times (95% CI = 1.4-19.6) more likely during rainy conditions than during dry periods (Fig. 1B) and the hazard of total brood loss increased by 5.0% (95% CI = 1.4-8.7) for each 1-day delay in hatching between 17 May and 12 August (Fig. 1C).

Figure 1 Fig. 1.   Predicted 30-day survival rates of mallard broods from proportional hazards regression model in relation to variation in (A) percent seasonal basins with ponds (WETSEAS), (B) rainfall (RAIN), and (C) hatch date (HATCHDATE) in prairie pothole landscapes in North Dakota and west-central Minnesota, 1988-94. For each variable shown, we held remaining explanatory variables constant at their mean values. Levels of HATCHDATE represent 10th percentile, median, and 90th percentile based on exposure days of mallard broods in our sample.

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