the effects of population sizes and water conditions, this variation may have 
been due as much to differences in abundance as to differences in habits. 
This possibility is given credence by the fact that the two infrequent dabbler 
species, baldpate and green-winged teal, make almost no use of small areas. 
The smaller, apparently less desirable, areas increase in importance as the 
breeding population of a species increases. 
Although detailed analysis of the effect of cover on the use of potholes 
by breeding pairs is impossible, it was found that under average conditions 
potholes with excessively dense vegetation were very little used.. Those with 
very sparse cover, or with no vegetation at all, were clearly preferred. On 
the other hand, a certain amount of cover was of value as wind shelter in cold, 
blustery weather. Birds were often noticed using shelter in such weather, 
while they almost completely deserted open wind-swept potholes. 
Other factors affecting the distribution of breeding birds should be 
mentioned. Although no intensive work was done at Waubay on food supply, in- 
spection of the potholes indicated abundant food in nearly all of them. Not 
only was a good supply furnished by vegetation, but invertebrates were partic- 
ularly abundant, even directly after the ice went out in the spring. To test 
the influence of food on the distribution of ducks in May of 1952, several 
bushels of wheat and corn were placed on the shore and in the water of a pote 
hole moderately used by blue-winged teal, gadwall, mallard, and pintail, and 
also frequented by most of the other species. None of this grain was touched 
by the ducks or even by the abundant blackbirds and mice in the area. Benson 
(1948), after careful analysis of available food and its use in a series of 
prairie ponds in Minnesota, also concluded that space requirements, rather 
than food, controlled the distribution of blue-winged teal. 
Land use, except for its effects on the vegetation of the potholes, has 
not been found to have any influence on the distribution of breeding pairs. 
Even the disturbing effect of farm buildings immediately adjacent to a pothole 
did not reduce its utilization. Intensive land use, involving the plowing or 
burning of the entire pothole, often improved it for pair use by removing ex- 
cess plant cover. 
It is improbable that water chemistry was a factor in the use of potholes 
by ducks. Although no samples were taken, the vegetation indicated that there 
were no extreme chemical variations. However, there were in the general region. 
a few relatively barren areas of high alkalinity which were little used by 
waterfowl. Owing to high water levels in 1952, one such area became highly 
attractive to ducks, and remained so in 1953, presumably because its high 
alkalinity was diluted. 
_ About the only water areas that were not used by ducks during a breeding 
season were those excessively overgrown or extremely alkaline. Because of 
changes in land use and water levels which modify these factors, even most of 
these are suitable during some years. 
Thus, the distribution of breeding ducks appears to be influenced pri- 
marily by a tendency toward dispersal, particularly in the case of the most 
abundant species. Hochbaum (1944) and Sowls (1951) discussed territoriality 
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