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Mallard Recruitment in the Agricultural Environment of North Dakota

Nest Habitat Preference

When use of a habitat is greater than its availability there is evidence of selection for that habitat (Johnson 1980). For waterfowl studies there is often a problem with defining availability. Waterfowl are so mobile that we may view our entire study area as available, but in reality the hen probably selects specific nest sites from a home range that was selected from the entire breeding range. Consequently, habitat within the home range is frequently defined as available habitat (e.g., Gilmer et al. 1975). Actually parts of the home range may not be available to the hen because of social interactions with other waterfowl whose home ranges overlap hers or for other reasons such as local disturbance. The home range of a bird determined from radiolocations is a function of number of locations used to define the boundaries (Odum and Kuenzler 1955). In our study the number of radiolocations was usually not adequate to define home ranges. We used, therefore, 2 specific definitions of land available for nesting: the entire study area and a 200-m-radius circle around each nest for which we had aerial photographs (Table 8).

Table 8. Relative preference by mallards nesting in central North Dakota obtained by comparing use of habitat at the nest with availability of habitat in a 200-m circle around the nest and within the entire study area.
Habitat at nest Percent use Available habitat (%) Preference ranka
200-m circle Study area 200-m circle Study area
Grassland 40.1 42.2 37.9 5 B 5 B
Wetland 16.2 13.2 14.8 3 B 4 B
Odd area 15.5 3.1 2.4 1 A 2 A
Right-of-way 14.1 4.3 1.3 2 AB 1 A
Hayland 12.0 11.7 5.1 4 B 3 B
Cropland 2.1 25.4 38.3 6 B 6 B
aHabitats with different letter symbols are significantly different (P < 0.05).

Under these definitions there are 2 possible comparisons of use and availability. Significance was tested by a multiple comparison, extending the method of Neu et al. (1974). Tests were applied to differences among relative preferences.

The relative preferences for habitats presented in Table 8 show that right-of-way and odd areas rank at the top in preference and cropland ranks at the bottom. The other types fall between these extremes, but our sample is not large enough to produce a consistent ranking pattern.

The results obtained from this type of analysis are, in part, a function of the habitat classification used. This problem is apparent in our data where grassland was used in about the same proportion as its availability. The plant community of grassland is diverse, and it is possible that birds selected a component of grassland. To test this hypothesis, we subdivided the area originally mapped as grassland in the 200-m-radius circle into grass and brush components. This new analysis of 7 classes showed a significant preference for brush and rejection of grass. The relative rank of brush was significantly lower than odd area and higher than grass and cropland. It did not differ from right-of-way, wetland, and hayland. Similar selection of 1 part of other classes such as wetland also may occur, but we were unable to test these because of problems in photointerpreting the available habitat.

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