90 H. BOYD 



men are more complex than those of commerical exploiters, but initially it 

 seems best to assume that their principal requirements are to have available 

 the largest possible number of full-grown individuals each season and to be 

 allowed to kill as many of them as they can without detriment to their 

 chances in future years. 



Very few wildfowl populations in Arctic and temperate regions of the 

 north are sedentary and many of them perform lengthy annual migrations. 

 Thus sportsmen shooting in the autumn and winter have, in general, no 

 direct means of influencing recruitment to the populations they are pursuing. 

 Moreover, these populations are often heterogeneous assemblies of birds 

 from many different breeding places. For a large safe kill shooting men 

 should favour those species rearing relatively large numbers of young and 

 particularly those in which a high rate of loss in the first year after fledging 

 is still consistent with the maintenance of the breeding population. But this 

 view will only be appropriate if losses from causes other than shooting are 

 small in these species, or if shooting losses can be substituted for natural ones. 



The mallard Anas platyrhynchos has been marked on a large scale in many 

 countries so that much information on the survival of different sub-popula- 

 tions is available. From a survey (unpublished) of this material I conclude 

 that the species is not capable of sustaining itself when adult losses exceed 

 about 55 per cent, although some authors (e.g. Hickey, 1952; Lauckhart, 

 1956) do not concede this. Self-maintaining populations of this species show 

 close similarities in their rates of recruitment and loss, despite great differences 

 in the shooting pressure to which they are subjected. 



The relation between the kill by man and other mortahty is not easily 

 investigated but in recent years the problems have received much attention 

 in North America and results are beginning to be announced. The most 

 impressive studies so far are those of the U.S. Bureau of Sport Fisheries and 

 Wildhfe on the canvasback Ay thy a vallisneria (Stewart, Geis & Evans, 1958; 

 Geis, 1959; Atwood, 1959). This is a valued game species subjected to heavy 

 shooting pressure, which accounts for more than a half of the deaths of all 

 canvasbacks of flying age. In North America shooting pressure is subject to 

 legislative control through the imposition of bag limits and seasons of varying 

 lengths and the Bureau studies have taken advantage of this situation to 

 demonstrate not only that shooting regulations have been effective in 

 influencing the size of the kill but also that the size of the kill has influenced 

 the size of the breeding population. This work is notable for dealing with 

 the entire wide-ranging specific population of the order of half a million 

 birds. 



The canvasback is an example of a species with a high turnover which 

 shows large changes in abundance from year to year, apparently due mainly 



