FLUCTUATIONS IN A RED GROUSE POPULATION 103 



less than, say, 10-20 per cent, but many alterations in population density 

 appear to be of the order of one-third or more (Table II) and are demon- 

 strated by this method of counting. 



In interpreting count figures it is necessary to compare data from the 

 different constituent parts with each other and with the total. An example is 

 given (Table III), taken from the third progress report (Jenkins, 1958^). 

 Fourteen counts were done between September and April 1957-8 and they 

 show that grouse numbers fell from 500-600 in the autumn to 370-390 in 

 the spring. 



Table IIL Some count figures between September 1957 and April 1958 



Count No. 



I 2 3 4 5 6 7 8 9 10 II 12 13 14 



Partn . 2ist 8th 30th i6th 26th — 8th 19th 29th nth 31st 22nd 3rd 9th 



Sep. Oct. Oct. Nov. Nov. Dec. Dec. Dec. Jan. Jan. Feb. Mar. Mar. 



Part III . 20th i2th 23rd 15th 27th 28th 7th 19th 29th loth 30th 21st 2nd 8th 



Sep. Oct. Oct. Nov. Nov. Nov. Dec. Dec. Dec. Jan. Jan. Feb. Mar. Mar. 



Note. — The figures are bracketed according to a possible interpretation of the data (see text). 



The data also suggest that the decline may have occurred in stages and 

 show that the count totals for parts of the study area change in a generally 

 similar manner. (Hence we are the more confident that the counting method 

 is reliable.j For example it is clear that on Part II the first two counts were 

 similar at about 190; the next two show a reduction to about 150; the next 

 five or six a further fall to 125-135; while fmally numbers fell to 1 10-120. 

 Similarly on Part III there was a first set of counts averaging about 170; then 

 a fall to about 135 maintained through six counts ; and fmally a progressive 

 reduction to 93. These instances could be repeated but they are summarized 

 in the total for 418 hectares (1,135 acres), of which II and III are a portion. 

 Clearly there were in succession three or perhaps four different levels of 

 numbers during the winter. 



Some counts were exceptions from the general trends. The figures for II 

 were fairly consistent except for count 7. On this particular occasion the 

 second half of the count was spoiled by the appearance of a hen harrier 

 {Circus cyaneus (L.)). It was not practicable to repeat the whole count and 

 only the second half was done again. This figure was then added to that for 



