32 
The total brood-days of use of the northern third with the fewest potholes was 
intermediate between the other two sections, and there was apparently no significant 
difference in the use per acre of pothole between the three regions, indicating that 
if pothole density is important, those in this particular area were all so closely 
spaced that it did not take effect. The influence of the spacing of potholes on 
brood mobility is quite marked. Those which occupied the region of high density 
were nearly twice as mobile as those in the northern third. It appears probable 
that the ease of access of more nearly adjacent potholes in the southern two-thirds 
might encourage movements. In support of this, it should be stated that the most 
mebile brood observed was the shoveller brood occupying potholes Nos. 29 and 31 
which .are separated by only a narrow strip of land. 
Food -- Although lack of time and facilities prevented a study of the availa- 
bility or utilization of food, it seemed probable from a casual inspection of pot- 
holes that food, principally invertebrates, was of such abundance as not to be a 
critical item... 
Two cases of rapid increase in the brood population of areas previously unten- 
anted may have been the result of a change in the abundance of food sufficient to 
attract broods. The first was pothole No, 10, used only for 8 days from July 23 to 
31 when it was occupied by six broods for an average of | days each. This may have 
been due either to a temporarily attractive food supply or to a chance concentration 
of broods which had increased mobility during this period of high water, The second 
case was pothole No. 35, which had no broods until July 12, when one came in. By 
July 13 there were six, and it was then occupied by from four to seven broods until 
the termination of the study, when six broods remained. 
Benson (1918) made a detailed study of the available food in a comparable area 
and concluded that it was so abundant that it was not an important factor in the 
selection of water areas, 
Occupancy by Other Broods -- This is a difficult factor to evaluate. Such data 
as were obtained indicate that no density limit was reached on the study area. 
The cases shown in Table 1) indicate the extent to which broods were known to 
associate in one pothole. These cases are well above the average concentration 
found on the study area and indicate that the population had not reached its density 
limit from the standpoint of broods, particularly since: there was no conflict 
observed which might have been attributed to high densities. ‘he mobility of 
these birds was calculated in the same way as the average mobility for the species. 
In only seven cases were closely packed birds more mobile than the average for the 
species. Apparently, the potholes in which the broods concentrated were more 
attractive than the average, and broods spent longer periods of time in them. All 
species but the ruddy had their greatest concentration in broods per acre in Type 
Al potholes. 
Use by Other Species -- Coots, horned and pied-billed grebes, and black terns 
were seen to occupy the same potholes as duck broods and at times appeared to be 
in conflict with them. Apparently there was no decrease in the use of potholes by 
ducks because of interference from these other species. The over-all average use 
of the )2 permanent potholes was-22.0 brood-days per acre of pothole. The 17 pot- 
holes with no coots averaged 10.2 brood-days per acre while the 25 which had coots 
average 30.0. The 29 potholes without grebes averaged 20.2 brood-days per acre 
while the 13 with grebes averaged 25.9. The 39 potholes without black terns averaged 
19.8 brood-days per acre while the three with black terns averaged 50.h. 
The only mortality apparently caused by these species was in pothole No. 1 
where several coots, both young and adult, and one half-grown mallard had been killed, 
