FLUCTUATIONS IN A PARTRIDGE POPULATION 



119 



autumn populations have also followed both high and low breeding densities 

 (cf. 1954 and 1959), and this emphasizes the point that the autumn population 

 is a product of the breeding stock and its success in producing and rearing 

 young birds. The number of pairs in the breeding stock is determined by- 

 four main factors: 



1. The autumn population in the previous year. 



2. The number of birds shot. 



3 . The number of birds that disappear between September and March. 



4. The sex ratio of the population prior to pair formation. 



4000 



3000 



2000 



1000 



1949 I950 1951 1952 1953 1954 1955 1956 1957 1958 1959 

 Fig. I. — March and September stocks, 1949-59. 



When the autumn population has been censused, the shootable surplus has 

 been determined each year on the assumption that approximately 40 per 

 cent of the unshot birds will disappear between September and March, and 

 that approximately 600 pairs (or a pair to 6 acres) is the maximum density 

 likely to be achieved in this type of country. Unfortunately, it has often 

 proved impossible to shoot the theoretically correct crop of partridges (due 

 to unfavourable weather conditions or field crop distribution) so that 

 shooting has not always been the accurate density dependent form of preda- 

 tion on the partridge population that it was intended to be (Table I, Fig. 2). 

 However, the number shot has varied from 10 per cent in low years to 

 3 5 per cent in years of relatively high recruitment and adequate breeding 

 stocks, and has resulted in December (post-shooting) populations of from 

 i»357 (1959) to 1,917 (1953)- (1958 has been excluded since, for experimental 

 reasons, the population was deliberately reduced to a low level in December.) 



In addition to the known stock reduction due to shooting, the partridge 

 population is considerably reduced between September and March (when 



