102 



DAVID JENKINS AND ADAM WATSON 



grouse are disturbed or not. Marked individuals are often useful in counts 

 and the fact that we flush them where expected gives us confidence that the 

 counting technique is reliable. 



It would be difficult to check the counting method by a capture-recapture 

 technique because it is not easy to catch a sufficient sample and because the 

 constituents of the populations apparently change frequently. The similarity 

 of repeated count totals suggests that we always count a similar high propor- 

 tion of the birds present. We believe that the mean value is often within 

 5 per cent of the actual number present and claim a general accuracy of plus 

 or minus lo per cent. We are satisfied that the technique is in any case 

 sufficiently accurate to show whether grouse numbers stay constant or 

 whether there are large changes. It may not show changes in numbers of 



Table II. Figmes obtained on counts, November igs^-Febxuary i960 



February- 



March 



April 

 May 



274 



274 



364 



379 



420-483 



361-417 



351-394 



Un- 



Old Young Total 

 333 658 991 

 476 



Old Young Total Old Young knownTotal 

 410 361 .771 . 205 89 71 365 

 134 56 



August . 

 Numbers shot , 



(approx.) 

 Notes 



1. 1958 was a late spring and an April count was possible. The May total for 1959 was derived 

 from many visits during late April and early May. 



2. Young birds were well-grown in August 1959 and it was often impossible to distinguish 

 them from old ones during the August count. The young to old ratio was 0-4:1 among the 

 known birds. Therefore the totals are estimated as no : 255 (the only reason birds were not 

 distinguished was that they flew off too far away from the observer). 



3. The figures for 1,135 acres for November and December 1956 are estimates based on the 

 empirical data from 830 acres (i.e. the figures for 830 acres multiplied by the mean ratio for the 

 January, February and March counts on 830 and 1,135 acres). 



