35 
VI. RATE OF MORTALITY AND DISPERSION OF BROODS 
Mortality 
Observed Losses -- Since recognizable broods were observed over a period of 
time there is some information available on the actual rate of loss of young. 
This is summarized in Table 16. The number of young observed (column 2) are those 
counted when the broods were first observed. 
The mobility of the broods from which these data were obtained were compared 
with the over-all mobility of broods as already discussed and was found to be the 
same, 
Table 16. Known losses from recognizable broods, 
er er o verage days Young 
Species broods young observed - ‘lost 
Canvasbac 11 . 3 21 0 
Raldpate 6 L6 31 6 
Redhead 3 21 32 6 
Mal lard 3 16 17 O 
Pinteil 2 13 28 1 
Ruddy 2 17 3k 2 
Blue-winged teal 1 10 h 1 
Shoveller 1 10 3h 2 
Gadwall 1 11 11 2 
Total 30 207 25 20 

‘It should be noted that there were no losses of canvasback or mallard, which 
together made up a large portion of the observations, while all but 2 of the 20 young 
which were lost were Class I birds indicating that losses decreased as the broods 
increased in age. 
Five nonmobile broods with 35 young were observed for an average of 27 days. 
When these broods were compared with 17 mobile broods of the same species which had 
107 young observed for an average of 27 days, the rate of loss of the former was 
found to be slightly greater than the latter. This is too small a sample to be 
highly significant but indications are that the broods which move overland may 
suffer less mortality than those which remain stationary and that possibly some 
movement may be to escape predation. 
Change in Average Brood Size -- Another way of measuring juvenile mortality 
is by means of comparing average brood sizes in the various age classes. Numerous 
authors gathered such data on the Canadian breeding grounds this same season. As 
reported in Williams (1919) table 17 shows the sizes of those broods for which good 
counts were obtained in the Minnedosa study. (Duplicate counts were’ eliminated 
wherever possible.) When each species was considered individually, there was an 
apparent increase in the sizes of broods as they became older. ; 
One explanation for this might be the combination of broods, or hens gathering 
up younp abandoned by other broods, but no indication was seen that either of these 
events took place. A more likely explanation is that first clutches, as reported 
by Sowls (1949), are significantly larger than those of later nesting attempts. 
The broods recorded as Class III during the term of the study were largely the 
products of these earlier clutches and may have been larger than broods hatched 
later which were recorded largely in the younger classes. Although the average 
size of all Class III broods is somewhat smaller than Class I, the difference in 
