114 DAVID JENKINS AND ADAM WATSON 



2. Total counts, using dogs to scent and flush grouse as the observer walks 

 standard areas, show that grouse have declined in numbers from the begin- 

 ning of the study in 1956 to a low level in early i960. Decreases have occurred 

 each winter through well-marked stages of loss (37-50 per cent and 24-43 per 

 cent), separated by periods of one to three months during which numbers 

 have stayed level. 



3. Marking of individuals and discovery of corpses have shown that during 

 these periods of stable numbers there have been changes in the individual 

 membership of the populations, those that died or emigrated having been 

 replaced by incomers from elsewhere. Thus the population density has 

 presumably been maintained level by some active process regulated through 

 the behaviour of the grouse themselves. 



4. Efficient exploitation depends on prediction of the levels at which the 

 grouse will stabilize their numbers at the end of the shooting season. 



REFERENCES 



Blank, T. H. & Ash, J. S. (1956). Marker for game birds. J. Wildlife Mgmt., 20, 328-30. 



Committee of Inquiry on Grouse Disease. (1911). The grouse in health and disease: being the final 

 report of the Committee of Inquiry on Grouse Disease. 2 vols. 512 and 151 pp. London. 



Jenkins, D. (1957, 1958^, 1958^, 1959, i960, 1961). ist and 2nd, 3rd, 4th, 5th, 6th and yth Pro- 

 gress Reports on the Scottish Enquiry into the Dedine of Red Grouse. (Mimeo.) Scottish 

 Landowners' Federation, 26 Rutland Square, Edinburgh i, Scotland. 



DISCUSSION 



I. A. McLaren: Was there any evident difference in the amount of 

 winter cover available for grouse between 1958 and i960 that might account 

 for the difference in population ? 



D. Jenkins : There was no evident difference as far as we could assess it. 



E. Duffey: How do the food resources of recently burnt heather 

 compare with those provided by old — say eight year old — stands ? Could 

 the pattern of burning cause variations in population level ? 



D. Jenkins: The information available shows that young heather is 

 more nutritious than old — at least to sheep. But I doubt if there is enough 

 year-to-year change to explain the very marked population changes. Cer- 

 tainly this factor is unlikely to operate in September, which is the time of the 

 most sudden population decline. 



T. H. Blank: Would you elaborate on your statement that 'more birds 

 are available than are present on the area'. 



D. Jenkins : We have found that numbers remain constant week after 

 week within our counting error of 10-15 per cent, although there is a known 



