132 DAVID JENKINS AND ADAM WATSON 



variables (e.g. rainfall) which appear poorly correlated, and picking on those 

 which are. 



2. That if the hypothesis (about sunshine) had been framed a priori, the 

 estimated correlation coefficient is sufficiently high to be just significant. 



A. Watson: In your abstract you state that predation on the young 

 birds may be a factor. What evidence is there for this ? 



T. H. Blank: There is very little real evidence. Of course it is known 

 that predation destroys some nests, and also we have records of the numbers 

 of predators trapped on the estate. But the latter do not give a satisfactory 

 indication of predator abundance, and no direct work on the predation of 

 chicks once they have left the nest is possible. There is no evidence that 

 predation is a serious factor in the mortality of these chicks, and in the study 

 area the population of predators is certainly too well controlled for its 

 year-to-year variation to account for the observed variation in the total 

 partridge population. 



E. D. Le Cren: What do you suppose would be the result if there was 

 no density-dependent shooting? 



T. H. Blank: Unfortunately we have not been able to make a test 

 experiment on this point. But we did once shoot very heavily on a low 

 autumn population, leaving between 1,300 and 1,400 birds after shooting. 

 Winter mortality accounted for a normal percentage afterwards, so that a 

 low spring breeding stock followed. Thus in our experience breeding stock 

 is related to the amount left after autumn exploitation; within certain limits 

 the more birds there are left, the more breeding pairs we should expect. 



D. Jenkins: I once carried out population work on partridges in 

 Hampshire for four years and reached rather different conclusions. My 

 results suggested that an important factor governing the number of pairs on 

 the area was the crop distribution in February. There was a constant breeding 

 population of partridges each spring, irrespective of the numbers of birds 

 left at the end of the shooting season, but associated with a constant pattern 

 of winter crops. In some cases the level of the spring partridge stock was 

 reached through the emigration of birds that were apparently surplus in the 

 late winter, but in one case a stock that was small at Christmas was increased 

 by birds immigrating to the study area in the late winter. Blank and Ash's first 

 graph suggests that in their study area the variation in the number of breeding 

 pairs is relatively minor in scale and that it too may be related to cropping 

 changes. It suggests that if there were no shooting there would still be little 

 variation in the breeding stock. Clearly if major mortality causes are removed, 

 such as those which might limit a stock through nest predation, the popula- 

 tion would be expected to rise to a ceiling but not exceed it. At Fordingbridge 



