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Comparison of Historical and Recent Prairie Waterfowl Nesting
Data to Determine Trends in Nest Success


Department of Zoology, University of Guelph, Guelph, ON
N1G 2W1, Canada

It has been hypothesized that (1) nest success is a "bottleneck" limiting growth of duck populations in the Prairie Pothole Region of North America and (2) increased predation pressure resulting from degradation of upland nesting habitat by agricultural practices has led to insufficient nest success to maintain certain waterfowl populations. It has been assumed that this trend can be reversed and that nest success, and thus population size, can be increased by reducing losses to nest predators through management techniques which deter predators. Before the effectiveness and cost-benefit of these management plans can be assessed, it needs to be determined if nest success has in fact declined in the prairies in the past few decades. We investigated historical trends in nest success of prairie ducks by analyzing existing data from the literature. Until recently, comparative studies of this type were difficult to undertake because incomparable methods of estimating nest success have been used in different studies. However, Green (1989) proposed a transformation procedure which can be used to convert the conventional "apparent" nest success estimators to "Mayfield-equivalent" success values, which can then be compared with actual Mayfield nest success estimates of more recent studies. We have calibrated this conversion equation using data from two published studies on waterfowl which report both apparent and Mayfield estimates determined from original nest search data. We found that Green's equation introduces little error (5 %) into the conversion of apparent nest success to Mayfield nest success. This error is less than both (1) the difference between apparent nest success and Mayfield nest success estimates when no transformation is made (15%) and (2) the magnitude of the presumed historical decline in nest success. Preliminary results of our analysis of published data of nest success are presented. It is important to consider that even under "ideal" conditions, some nest loss is inevitable and it is necessary to ascertain what the "normal" rate of nest success is before trying to decide to what extent it can be improved upon by management of predators.
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