31 
the first of 1.5 inches starting on July 19 and the second of 2.9 inches starting 
on July 27. They indicate that nswement of canvasback broods may be stimulated 
either by high water or by rising water levels. 
This method of analysis could not be used for other species since either there 
were too many broods hatching or flying during the critical period or there were 
too few broods present. 
Effect of Depth and Season -- Since the depths of potholes were variable and 
the use of different areas was seen to vary with the season, four of the more 
mobile species, two diving ducks (redhead and canvasback) and two river ducks 
(mallard and pintail), were considered with regard to their depth preferences. 
Owing to their high mobility, these species were able to respond most easily to 
changing conditions. 
Plate XVI shows the use made of water areas of several types in two depth 
classes. The measurements were made in mid-August and are considered sufficiently 
accurate for such a comparison. It can be seen at a glance that the diving ducks 
made more use of the deeper areas than did the river ducks. 
Mose of the use of A} areas by diving ducks occurred early in the season. 
This may be due in part to their abundance of submerged aquatics which later 
developed to such an extent that they made diving difficult. Both groups of broods 
showed a slight drop in the use of shallow areas during the dry periods of mid-July 
and late summer, and an increase in their use during the high water from July 19 to 
early August. It should be remembered that, as in the data shown in Plate XV, there 
is room for an error of 2 or 3 days in estimating the times when these broods were 
in each pothole, which might obscure rapid shifts in use, The rise in use of deep 
areas by mallard and pintail broods may actually have contimed until July 19 and 
there may have been an increase in use of shallow areas starting on that date. 
The decline in the use of deep areas by redhead and canvasback broods may also 
have occurred as a result of the rain on July 19. This serves to indicate that 
there was very probably some shifting of the general population due to falling water 
levels as was found to occur in pothole No. 33 where a daily count of the broods 
present (Plate XVII) shows a steady drop from 10 broods to 3 during the period when 
water levels were falling (July 8 to July 19). During this time the water was 
receding from the shore cover so that broods could no longer swim freely through 
the vegetation. At the same time the pothole became quite shallow (18 inches at 
the deepest point) so that broods could not easily escape by diving. Both of these 
conditions, which did not occur to such a marked degree on any other pothole, may 
well have made them nervous and encouraged movement away from the pothole. 
Broods apparently react to unfavorable conditions be deserting an areas, and 
may reoccupy it when conditions again become favorable as they did in this instance. 
Abundance of Potholes -- A glance at the map of the study area (Plate III) shows 
that roughly its northern third has fewer potholes than the remaining two-thirds. 
The effect of this difference is shown in Table 13, which is based on permanent 
waters only. 
Table 13. Effect of abundance of potholes on use by broods and 
mobility of broods. 
Northern 1/3 Middle 1/3 Southern 1/3 
Number of potholes 11 17 16 
Brood-days of use 756.0 ‘ 601.5 957.0 
Brood-days of use/Acre 
of pothole 26.3 622.8 26.7 
Mobility of broods 76 15.4 12.8 

