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Mallard Recruitment in the Agricultural Environment of North Dakota

Habitat Conditions and Climate

Aerial photographs were taken of a systematic sample of 66 1 x 3.2-km plots twice during each year, except 1978 when 4 flights were made. Thirty-five-mm format was used in 1977 and 70-mm format in 1978-80. Scale ranged from 1:18,000 to 1:28,000. Pond basins on each plot were classified according to Stewart and Kantrud (1971) by ground study, and a map was prepared for each sample plot. Aerial photographs were interpreted after each flight, and each pond was placed in 1 of 3 wetness classes of nearly full, about one-half full, or almost dry. These data were used to develop wetness indices for each wetland class (Cowardin et al. 1981).

Upland habitat on the transects and at nest sites was classified into 1 of 6 cover types as follows:

  1. Grassland included all pasture and idle areas dominated by native or planted grasses. Native grasslands usually had inclusions of woody vegetation, primarily western snowberry (Symphoricarpos occidentalis) or silverberry (Elaeagnus commutata).

  2. Hayland included all areas planted to forage crops and mowed for hay. Nearly all haylands were planted to alfalfa (Medicago sativa) or yellow sweetclover (Melilotus officinalis).

  3. Wetland included all areas within the wet meadow, shallow marsh, deep marsh, and fen zones defined by Stewart and Kantrud (1971).

  4. Cropland included all areas that were annually tilled for production of grain and row crops.

  5. Right-of-way included the area between fences and the road surface or between cropland borders and the road surface along primary and secondary roads and railroads.

  6. Odd areas included an array of different features such as rock piles, haystacks, gravel pits, shelterbelts, and farmsteads.
Data from the photographs were used to estimate available habitat within the study area.

Vertical aerial photographs centered over each nest and at an approximate scale of 1:12,500 were obtained as soon as possible after termination of the nest. These photographs were enlarged to a scale of 1:2,400, and the cover types were photointerpreted and delineated. Photointerpretation was verified during later visits to the nest. A circle with radius of 200 m was inscribed on the photographs (Fig. 2). The area of each cover type within the 200-m-radius circle was delineated and measured with a Numonics Graphics Calculator (Model 1224, Numonics Corporation, 418 Pierce Street, Landsdale, PA 19446óReference to trade names does not imply government endorsement of commercial products).

JPG-aerial photo
Fig. 2. Aerial photograph of mallard nest site (at center of small
circle) in a semipermanent wetland. Field on left is swathed grain.
Several rock piles (odd areas) occur in this field. The field to the
south is summer fallow (cropland). The one on the right is native
grassland (grassland). Small numbers refer to a detailed classifi-
cation used in field work but not described in this paper. The
large circle is 200 m in radius and defines bounds of area used
for analyzing habitat availability.

We also inscribed 1-km-radius circles around nest sites on overlays of 1:63,360-scale color-infrared photographs. Within the circle we determined the number of wetlands by size class, the distance from the nest to the nearest wetland at time of nest initiation and time of hatch, the distance to the nearest building, and the distance to travel lanes (e.g., roads, fence rows, shelterbelts) that might be used by predators.

When nests were first visited, the 3 most abundant plant species at the nest site were listed. Density of vegetation at the nest site was measured by the obstruction of visibility of a pole with markings painted at 5-cm intervals as described by Robel et al. (1970) and later modified for waterfowl studies (Kirsch et al. 1978, Higgins and Barker 1982). Ground photographs were taken of each nest and were used later to classify the nest site by vegetative life form.

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