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Factors Limiting Mallard Brood Survival
in Prairie Pothole Landscapes

Introduction


Waterfowl managers need effective and economical means of estimating annual duck production on managed and unmanaged lands in public and private ownership. For mallards, brood survival (measured here by total brood loss) is one of the most critical but least understood components of recruitment (i.e., production of fledged young [Johnson et al. 1992]). We sought to identify key factors influencing brood survival and to develop a model that could serve to predict brood survival rates. We focused our research on sites in the Prairie Pothole Region (PPR), the principal breeding grounds of mallards in North America (Anderson and Henny 1972).

We selected environmental factors to study using clues or questions raised by previous research and taking into consideration ease of measurement. Past studies indicate that the ratio of immature:adult mallards in the harvest is positively correlated with number of ponds in the PPR (Crissey 1969), and that brood survival is related to wetland density (Rotella and Ratti 1992). Fragmentation of perennial cover is known to be a major cause of low nest success in mallards and other dabbling ducks (Greenwood et al. 1995), but little is known about its possible effects on brood survival. Percent of upland landscape in perennial cover varies spatially across the PPR due to topography, soils, conservation programs, and other factors.

Although there is little information on the influence of weather on total brood loss in mallards, inclement weather has been shown to lower survival of canvasback (Aythya valisineria) ducklings (Korschgen et al. 1996) and probably affects duckling survival of many other species, either directly or indirectly (Johnson et al. 1992). Mallard mortality is greater in younger ducklings (Ball et al. 1975, Talent et al. 1983, Orthmeyer and Ball 1990, Rotella and Ratti 1992) and increases with hatch date (Orthmeyer and Ball 1990, Rotella and Ratti 1992, Dzus and Clark 1998). Older, more experienced females may be more attentive to broods, thus improving their chances of survival. Brood size also might affect brood survival rates if larger broods are less likely to suffer total loss through attrition.

In short, several biotic and abiotic factors may influence survival of mallard broods. Our objective was to identify factors affecting survival of mallard broods in prairie pothole landscapes that managers could easily measure and use to predict brood survival rates. We developed a model that included such factors but also included other less-easily measured variables in order to control for their effects. Specifically, we evaluated effects of water conditions, upland cover, ambient temperature, rainfall, hatch date, brood age, age of brood female, and brood size on mallard brood survival.


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