Northern Prairie Wildlife Research Center
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:
- 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).
- 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).
- Wetland included all areas within the wet meadow, shallow marsh, deep
marsh, and fen zones defined by Stewart and Kantrud (1971).
- Cropland included all areas that were annually tilled for production of
grain and row crops.
- 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.
- 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 19446—Reference
to trade names does not imply government endorsement of commercial products).
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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