of young to adults in hunters! bags will exceed that found in the 
population so long as this ratio among birds alive is a changing 
and shrinking one. 
Table 68.—~Some Potential Age Ratios for Mallards in 
Hunters! Bags’ 
This table illustrates some age ratios to be expected in hunters! 
bags over an entire hunting season when a ratio of 1.6 young per 
one adult in the population on September 1 is reduced to 1 young 
per adult by February 1. A second assumption is made cn colum 
2 for each line regarding the percentage of adults dying from 
hunting; from these the other figures follow, 
(1) (2) (1) = (2) (3) (4) (3) = (h) 
% of Age Class Shot Relative Vilnera- Relative Nos. Shot Age Ratio in 
oung u bility Over from 160 from 100° fhinters' Fags 
Entire Season Young Adult Me. ver 1 Ad) 
46.9 15 301 75 15 5 
50.0 20 2.5 80 20 hi 
53.1 25 2.1 85 25 304 
56.2 30 1.9 90 30 3 
59.3 35 17 95 35. 267 
62.5 O 1.6 2.5 
100 Lo 

This principle is well summed up in table 69. In this 
tabulation I have included an age ratio necessary to balance the 
mortality rates recalculated in table 58 for Illinois-~banded reported 
on by Bellrose and Chase. Although their work covers a relatively 
short period, the result gives a fairly consistent picture. (I 
interpret their work as holding for late autumn since a critical 
correction factor used in their calculations is based on birds shot 
in the latter part of the hunting season.) 
Two or three important variables govern aze ratios in 
waterfowl and deserve mention. In the first place, differential 
migratory behavior can easily distort age ratios as indices of 
population structures; at A. S. Hawkins! request I reviewed the band- 
ing data on this point in 197 but found the results to be incon- 
clusive (not enough mallards had been banded as young at that time). 
The phenomenon is widely suspected but as yet little understood. 
A second variable is the vulnerability of young birds 
to hunting (see coluwm (1) + (2) in table 68). Is this a gradually 
or an erratically decreasing phenomenon during the autum? I. 
gather that it may well be erratic due to the great differences 
in the opening dates of hunting that occur between the various 
states and provinces. | 
160 
