122 



T. H. BLANK AND J. S. ASH 



Subsequent increases in the acreage under grass were accompanied by 

 reductions in late winter loss. 



Another factor which may modify the late winter loss figure is the sex 

 ratio of the birds in coveys prior to pair formation. While methods for 

 determining the sex ratios of partridge populations will be discussed else- 

 where, the young/old ratios in August, the sex ratio shown by the shot 

 birds, and the September and December censuses form the basis for these 

 figures. In addition to the sex ratio of shot birds, in most years further 

 evidence has been obtained from the examination of live birds netted in 

 September and October. Each year there has been an excess of cock par- 

 tridges. Among juveniles the excess is usually very small (except in the years 

 1953 and 1955 when the cock/hen ratio of juveniles was 156/100 and 135/100 

 respectively), and excluding 1953 and 1955 has averaged 103 cocks to 100 

 hens. Adults show a very marked preponderance of cocks, and the sex ratio 

 from 1949-59 has averaged 157 cocks to 100 hens. Obviously, the overall 

 sex ratio depends on the young/old ratio of the December population, and 

 this has shown considerable variation in the different years. The proportion 

 of the late winter loss which may be attributed to unbalanced sex ratios has 

 averaged (1949-59) 27-6 per cent, and varied from i per cent in 1949-50 to 

 58 per cent in 1953-4. Some of these surplus cocks do remain on the estate, 

 and there is a direct correlation between the calculated surplus cocks (Decem- 

 ber) and the observed single cocks in March (Table II). In addition to the 

 surplus cocks which leave the estate, it has been calculated that each year 

 between 150 and 250 hens disappear from mid-December to mid-March. 

 From observation of marked birds, it is known that this loss is partly due to 



Table II. Calculated sex ratios in December stock (post-shooting) and sex ratios 

 observed in March (following year) 



sex-ratio of shot birds and the August young/old ratio.) 

 * In this year adult cocks exceeded adult plus juvenile hens. 



