336 



Illinois Natural History Survey Bulletin 



Vol. 2j, Art. 2 



ability differences between the species 

 are partially or entirely nullified, and a 

 measure is derived for the influence of 

 the open season. The shooting pressure 

 quotient can be changed for any species 

 by altering the opening or closing dates 

 of the season. 



The relative vulnerability, or killa- 

 bility, of species is summarized for a 

 5-year period, 1938-1942, in table 2. 

 A period of this length, rather than a 

 shorter period, was used because we be- 

 lieve there is little difference from year 

 to year in the inherent wariness, or in 

 the flight, flocking and feeding habits, 



changed materially from year to year in 

 the period 1938-1942. 



Vulnerability of Species 



Because of variations in habits, spe- 

 cies of ducks differ in their likelihood of 

 being killed by hunters. Species differ 

 in inherent wariness. Food habits in- 

 fluence the killability of species; pref- 

 erence for a certain food may entice one 

 kind of duck into heavily gunned areas, 

 while another kind remains in open 

 water, out of reach of hunters. Flocking 

 habits also influence the vulnerability 



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 CVJ 



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 CM 



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Fig. 5. — Chronology ot flight of all duck species, and species other than mallard, bag per 

 hunter per day and total daily duck kill (weekly averages expressed as per cent of yearly total)] 

 in the Illinois River valley, 1940. • 



of a given species. However, in evalu- 

 ating the comparative mortality of duck 

 species, we used yearly data, tables 3-7, 

 because the hunting regulations and 

 chronology of migration — which may 

 greatly influence the kill of a species — 



of species, for, generally, singles are; 

 more readily decoyed than a flock of 10, 

 which in turn is more readily decoyed 

 than a flock numbering 100. As figs. 

 5-7 show, the greater the concentration 

 of ducks, under ordinary conditions, the 



I 



