124 T. H. BLANK AND J. S. ASH 



Survival of young birds is measured in two ways. A series of sample counts 

 of adult and young birds in early August provides figures from which an 

 annual young/old ratio is obtained. While this ratio may be affected by loss 

 of adult birds between March and August, the ratios should have comparative 

 value since this loss has been fairly constant (15 per cent (±4 per cent) of 

 March population). Summer gain may also be assessed by expressing the 

 September census minus the March census as a percentage of the March 

 population. This figure allows for any adult loss between March and 

 September, but since September censuses have not been carried out on any 

 other estates, comparable figures are not available for different parts of the 

 country. If no loss of adults occurred between March and September, and 

 if no juvenile disappeared between mid-August and mid-September, then 

 percentage gain and young per 100 adults should be the same figure. 



Whether we consider the young/old ratio in early August or 'percentage 

 gain' figures taken a month later, the annual variations from the eleven-year 

 average (1949-59 young/adult pair ratio = 3-83 + 4*5i/— 2-73) are very 

 marked. And since there appears to be little or no connection between any 

 of the variable factors in the pre-hatching period with the subsequent young/ 

 old ratios, correlation with variable environmental factors effective during 

 the post-hatching period must be looked for. Of the many factors which 

 might exert control over chick survival rates, weather is perhaps the most 

 obvious. Unfortunately, we cannot measure weather as a whole — only the 

 many different aspects such as sunshine, rainfall, etc. can be separately 

 recorded, and no single index representing weather conditions can be ob- 

 tained. Further difficulties arise from the fact that many weather measure- 

 ments are discontinuous (often only minimum and maximum daily readings), 

 and that birds experience actual weather conditions and not the monthly 

 average. It is quite possible that conditions of temperature and rainfall that 

 are lethal to young birds may be masked if only monthly averages are 

 available. Thus in 1953 a three-day period of Ught continuous rain combined 

 with relatively low day temperatures, appeared to decimate brood-size, 

 although the average rainfall for the month was normal. The date of onset 

 of adverse weather in relation to the peak hatching period is also of import- 

 ance. This period has varied from the loth to the 19th June, and bad weather 

 occurring before these dates is less effective in reducing brood size. However, 

 in spite of these qualifications, there is a close correlation between the 

 recruitment rate (as measured by the young/old ratio) and the weather 

 (average rainfall, sunshine, etc.) during June and July. The highest rates have 

 occurred when these months have been most dry and sunny, and the lowest 

 recruitment has occurred when June and July have been wet and relatively 

 sunless (Table IV, Fig. 3). Further evidence of the effect of weather during 



