Critical Review of the Aerial and Ground Surveys of Breeding Waterfowl in North America




The U.S. Fish and Wildlife Service and the Canadian Wildlife Service in cooperation with others have conducted an annual survey of breeding waterfowl throughout central Canada, the north-central United States, and Alaska since 1955. The area comprises more than 50 strata of habitats. Ducks are counted from aerial transects, and the counts are adjusted upward to account for birds that are not observed by aerial crews. These adjustments, called visibility factors, are developed from counts on the ground during which all waterfowl are assumed to have been detected. Counts on the ground are made of a subsample of the aerial survey. Visibility correction factors are calculated for each species and for each aerial crew. The total number of ducks by species and by strata is then calculated as the product of the observed density, the visibility correction factor, and the area of the strata. A problem arises if the aerial or ground crews count an insufficient number of individuals of some species on the visibility portion of the survey in a year. A minimal count is needed before the visibility correction factor can be reliably calculated. When insufficient birds are seen in a given year, the data from previous years must be used for the calculation of the visibility correction factor.



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