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Determinants of Breeding Distributions of Ducks

Introduction


The settling of habitat by migratory birds within their geographic range represents habitat selection on a coarse scale (Johnson 1980), finer than geographical range, but broader than home range and microhabitat. Analysis of settling behavior on this scale raises both biological questions and management issues.

On the biological side, there is a considerable body of theory concerning geographic distribution of highly mobile animals such as birds. Both the ultimate factors that influence evolutionary fitness in the habitat selected and the proximate cues used by animals to select habitats have been intensively studied. Life history characteristics of different species have been related to habitats occupied by the species; much of the discussion on this topic has been cast in terms of r- and K-selection (Pianka 1974). Distributions and basic biology are particularly well studied in ducks (Johnsgard 1975, Palmer 1976, Bellrose 1980), thus providing opportunities for detailed analysis.

The nature of settling patterns of ducks is important to waterfowl managers at both continental and local levels. To regulate hunting, for example, managers predict the size of the fall population by combining estimates of the breeding population with measures of productivity (Martin et al. 1979). Because reproduction varies widely among breeding grounds (Hansen and McKnight 1964, Calverley and Boag 1977, Derksen and Eldridge 1980), understanding how ducks distribute themselves throughout their range should provide useful information for predicting the size of fall populations. At a local level, the effectiveness of land management techniques to increase numbers and production of ducks depends on the extent to which ducks return to the same area in subsequent years or seek out new breeding locations (Hochbaum 1946).

Our objectives were to determine ecological and geographic correlates of the breeding distributions of 10 common species of North American ducks and to relate these to the life history characteristics of the species. The species are mallard, gadwall, American wigeon, green-winged teal, blue-winged teal, northern shoveler, northern pintail, all of which are dabblers (Tribe Anatini), and canvasback redhead, and lesser scaup, all divers (Tribe Aythyini). This analysis is restricted to the area in North America regularly surveyed for breeding ducks (Fig. 1), and data used are results of those surveys.


gif -- Strata locations used in survey

Figure 1. Strata used in breeding waterfowl surveys.


We propose 3 patterns that migratory birds could use for settling breeding habitat. We then describe the methods used to gather and analyze the data. Distributions of breeding ducks are described and related to wintering areas and migration corridors, local and continental wetland conditions, and homing and pioneering tendencies of each species. We also describe relations among the species and how our findings fit with the species' life history characteristics. We conclude with some implications for management.

Acknowledgments.óWe are grateful to R. S. Pospahala and others at the Office of Migratory Bird Management, U.S. Fish and Wildlife Service, for providing survey data used in the analyses, and to A. D. Afton and R. A. Wishart for use of their unpublished information on homing of lesser scaup and American wigeon, respectively. We appreciate the valuable comments on various drafts of this report made by M. G. Anderson, F. C. Bellrose, H. Boyd, W. F. Crissey, A. Dzubin, J. L. Eldridge, R. R. Koford, G. L. Krapu, J. T. Lokemoen, H. W. Miller, J. D. Nichols, T. D. Nudds, G. L. Nuechterlein, and A. B. Sargeant. M. R. Miller offered numerous suggestions that improved the manuscript.


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