FLUCTUATIONS IN A RED GROUSE POPULATION 115 



mortality and exodus. Presumably therefore there is a surplus of birds flying 

 in from other habitats and replacing those dying on our area. 



T. H. Blank: There must be a compensating increase in populations 

 elsewhere at times when your populations show sharp declines. 



D. Jenkins: Not necessarily; birds leaving one area may fmd another 

 place that is temporarily understocked and in this case they will settle and 

 the numbers in the second place will increase. But if the emigrating birds fail 

 to fmd an understocked area, they may scatter among the marginal, more or 

 less uninhabitable, places interspersed between the optimum habitats. Here 

 they will eke out a living through constantly paying short visits to the 

 optimum habitats, seeking a territory vacated by a previous resident. The 

 mortahty-rate in the marginal areas is probably very high, and the surplus 

 birds may rapidly be lost through disease, starvation, predation and so on. 



G. C. Varley: A consideration of the possible movements of birds leads 

 me to wonder if Jenkins and Watson may have chosen a really favourable 

 habitat for grouse for their census: an area selected preferentially by them. 

 At the end of the season birds would be expected to scatter, to be displaced 

 from such an area, returning from less favourable surrounding areas later to 

 make up gaps left by mortality in the study area. Thus the autumn fall in 

 the study area would be balanced by a rise in the adjoining unfavourable 

 habitats. 



D. Jenkins : The study area which I described is only one of a series 

 which extends upward to the upper altitudinal hmit of grouse in the district. 

 A pattern of population changes similar to that on the lower ground is 

 observed under these higher conditions. This suggests that all habitable areas 

 are therefore in a sense ^saturated'. Even a sample area 50 miles away, where 

 grouse are at different levels of population density, shows a comparable 

 pattern of changes, as if a large area of eastern Scotland was similarly affected 

 by a bird which everywhere over-reproduces itself, and is usually under- 

 exploited. 



G. C. Varley: But surely the stability of population in the study area 

 can only arise because of immigrants coming in from outside. The source 

 areas for these birds must not have, stable populations. Where do these 

 immigrants come from? 



D. Jenkins: I think there are two possible sources. First, there is often a 

 surplus of birds from areas like ours spreading in autumn and late winter 

 trying to colonize new areas and many of these may fmd suitable niches 

 elsewhere. Secondly some surplus birds that fail to fmd optimum niches may 

 be present much of the year as non-breeders in marginal habitats, from which 

 they pass as transients through high-density areas. They would then be 

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