Differential Vulnerability 
Age and sex groups of all species do not have the same probability 
of being taken by hunters (Bellrose, 1944, 1950; Geis, 1959). Asa 
result, the age, sex, and species compostion in the hunting kill is 
not the same as the age, sex, and species composition in the waterfowl 
population. In order to use kill data for many purposes, it must be 
assumed that differential vulnerability is essentially constant each 
year. The extent to which this assumption is true is unknown at 
present. However, it is reasonable to conclude that variations 
between years in waterfowl production, for example, are probably much 
more important than variations between years in differential vulner- 
ability. Assuming this to be true, the observed age ratios could 
Serve as reliable indexes of production rates. 
Information on the age, sex, and species composition of the 
hunting kill would be much more useful if it could be adjusted for 
differences in vulnerability, so that the actual age, sex, and 
species composition of the population prior to the hunting season 
could be determined. 
Adjustment of age ratios probably is the most important aspect 
of this problem. In order to make this adjustment, it would be 
necessary to have an adequate sample of birds banded prior to the 
hunting season. Some banding was done prior to the 1959-60 hunting 
season (Table 61) , but not enough to permit making adjustments in 
the wing-collection data. Data from this banding, however, do 
suggest several possibilities and problems. Mississippi Flyway 
recoveries from birds banded in Manitoba, Minnesota, and North. Dakota 
suggested that immatures were approximately 1.4 times as vulnerable 
as adults to shooting. However, Mississippi Flyway recoveries from 
Saskatchewan bandings indicated that immatures were only 0.81 times 
as vulnerable as adults. The number of immatures banded in Saskatchewan 
was too small to permit a precise measure of the recovery rate of 
immatures, and therefore this result could have been due to chance. 
It is possible, howeyer, that adults were more likely to be taken in 
the Mississippi Flyway than immatures because of the greater tendency 
of the immatures to be harvested on or near breeding grounds and early 
in migration. Many of the immature birds banded in Saskatchewan were 
harvested before they reached the Mississippi Flyway wintering grounds. 
Table 61 shows that for all areas combined immatures were more vulnerable 
than adults. It has been shown for the canvasback (Stewart, Geis and 
Evans, 1958) and for the black duck (Martin, 1960) that the kill of 
* Due to complications resulting from the reconstruction of fire- 
damaged banding records, this information may be incomplete. 
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