The only requirement for nesting was the proper type of cover at the 
nest site. Other factors, such as the quality of the nearest pothole, were 
secondary. The availability of good nesting cover depended considerably on 
farming practices in the case of upland nests, and on water levels in the case 
of over-water nests. Both of these factors were highly variable. A pothole 
with sufficient nesting sites for a large number of pairs one year may, be- 
cause of changes in water levels, be entirely unsuitable for any nesting the 
next. On the study area, over-water nesting cover did not appear to be a 
critical item. The productivity of the two most abundant over-water nesting 
species, the redhead and mallard, increased in 1952 in spite of the reduction 
of hardstem bulrush nesting cover due to flooding. 
Broods preferred open-water areas and deep marshes, and there was a 
minimum of friction between broods even where there were heavy local concen- 
trations. 
In such a region, with its ample supply of “brood potholes" and nesting 
cover, it appears that the limiting factor is probably the space necessary to 
the spring dispersal of pairs. A shortage of this item is not measurable in 
terms of ducklings lost or eggs destroyed, but results in unlaid eggs, deserted 
nests, or perhaps hens careless in their nesting. 
As an illustration, once her embryo has started development, a female 
mammal will generally produce young, barring excessive malnutrition, disease, 
or serious accident. A bird, on the other hand, if psychologically or physi- 
ologically disturbed during egg laying and incubation, might leave her clutch 
at any time and end that particular nesting attempt. Once hatched, most of 
the yo will reach flying age, barring excessive drought or predation. Two 
studies (Bue et al., 19523 Evans et al., 1952) have shown that under near- 
normal conditions mortality of young is about 10 percent. Under similar con- 
ditions, nest mortality may be expected to be considerably higher. The period 
when the habitat may be expected to have the most influence on reproductive 
success is the early spring when the pair is together and for the following 
few weeks while the bulk of the hens are. incubating. At Waubay, during the 
four years of the study, this has been from early May through most of July 
(figs. 12-13). Figures 9-10 show that this has been the period when the 
greatest number of potholes were filled, and the carrying capacity for pairs 
was at its peak. 
EFFECT OF THE PRESENT DRAINAGE PROGRAM ON DUCK PRODUCTION 
Drainage program 
Drainage on the Waubay study area and in similar country is now a matter 
of draining potholes from a relatively high elevation into lower-lying basins; 
very little of the water actually leaves the vicinity. Seldom is the drained 
pothole as large as the one that receives the water, and the average size of 
the 39 that have been drained under the present program is 0.7 acres. Figure 
22 shows the pattern of drainage on a part of the Waubay study area. A few 
additional potholes were drained or partially drained, before the present 
subsidy-payment program, which began in South Dakota in 1944. 
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