in waterfowl and concluded that the desire for isolation from other members 
of the same species is an important factor during the breeding season. It 
appears that this desire for isolation in the pothole country leads to a 
dispersal of the population and brings about intensive use of parts of the 
habitat which are not otherwise used. 
Distribution of nests 
Although the work at Waubay did not involve intensive nesting studies and 
the sample obtained was probably not representative, 294 nests were recorded. 
Of this number, 95 were in the emergent vegetation of potholes. These include 
48 of the 86 mallard nests and all but 2 of the 48 diving-duck nests. Hardstem 
bulrush was by far the preferred cover for these species; cattail, whitetop, 
and other plants contained only a few nests. 
Over-water nests were distributed fairly evenly throughout the pothole 
sizes above 1/2 acre, while only one nest was found in a smaller area. Depth 
and water-level fluctuations are definitely the important factors in con- 
trolling the emergent vegetation, and hence the selection of cover for nesting. 
The great majority of these nests were found in either shallow or deep marshes 
which go dry from time to time and which had the best developed growth of 
emergent vegetation. 
Forty (42 percent) of all over-water nests found were in three deep 
marshes. All of these were found in 1950, 1951, and 1953, since in the high- 
water year of 1952 the emergent vegetation of these areas was completely 
flooded out and unusable for nesting. Most suitable nesting cover in 1952 
was found in areas with overflows, which controlled water levels and prevented 
flooding, or in a few potholes where levels had previously been below normal. 
The remaining 199 nests, all of puddle ducks except two of lesser scaup, 
were scattered throughout available cover and showed a slight tendency to 
cluster in the vicinity of potholes. This clustering tendency was not partic- 
ularly significant, since in this type of country there were few places for 
ducks to nest which were more than 300 yards from water. Nesting in the 
bottoms of dry potholes or in the pothole margins generally occurred in areas 
of medium or Low permanence, where the rapid drop in water levels made such 
cover available for dryland nesting early in the spring. 
It is probable that the majority of successful upland nests were in 
standing small grain which covered 63 percent of the study area. Local resi- 
dents have told of finding a nest for every 15 acres when they swathed the 
grain in August. Considering the size of the breeding population and the num- 
ber of broods produced, as well as the late date, this is a high concentration 
of nests. Furthermore, although the intensity of censusing for upland nests 
was fairly constant throughout the season and although a hunting dog was used 
two years of the four, very few nests were found after July 10. Referring 
back to figures 12 and 13, and allowing roughly 30 days for laying and incu- 
bation, less than 40 percent of all successful nests had hatched by July 10 
in the four years of the study. Standing small grain is the only type not 
censused and becomes available as nesting cover about July 1. It thus appears 
certain that the majority of late-season broods of upland-nesting species were 
hatched in this type. 
39 

