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Breeding Population Inventories and Measures of Recruitment

IV. Sampling

B. Stratification

Waterfowl productivity and breeding density can vary greatly in any survey region, and stratification usually increases the precision of sample estimates (Rutherford and Hayes 1976). A number of criteria have been used for delineating strata. The cooperative breeding ground surveys use large strata selected on the basis of prior knowledge of breeding waterfowl density and habitat characteristics. The strata are bounded by political boundaries, states and provinces, and are subdivided according to latitude, pond density, and physiography. Isakov (1970) divided the Soviet Union into districts and stratified these based on prior knowledge of waterfowl density. Stewart and Kantrud (1972) used geographic variation in species composition within physiographic regions as a basis of stratification in their studies of breeding waterfowl in North Dakota. In their first studies of breeding waterfowl in the prairie pothole region, Stewart and Kantrud (1973) used a simple stratified random sample. That sample proved inefficient because of the amount of travel between sample plots. Later they (Stewart and Kantrud 1974) used two-stage cluster sampling. Plots (64.8 ha) were drawn at random from randomly drawn townships (93.2 km²) . Reed and Changnon (1987) stratified their study area by using habitat maps and aerial photographs. They created three strata representing perceived potential for production of greater snow geese and then used optimum allocation to assign plots to the strata. Chamberlain and Kaczynski (1965) stratified their sample of aerial transects after the fact based on phenological events such as the date at which ponds and lakes became free of ice.

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