ii6 DAVE) JENKINS AND ADAM WATSON 



available as colonists for optimal places. Most birds in marginal habitats 

 probably die very quickly and these populations must be very unstable. 

 Dying birds are certainly sometimes seen, even in the breeding season, in 

 unlikely places hke farmyards, grasslands and other areas which are not 

 normal grouse habitats. Perhaps lo per cent of the total population may 

 sometimes be made up of these surplus dying birds. If the population is 

 rising, presumably such birds are likely to become estabhshed. At other times 

 when they try to inhabit marginal habitats where they do not breed, they 

 are likely to die, perhaps through starvation or predation. 



I. A. McLaren: Surely you may have a long-term trend, like that 

 between 1956 and i960 and yet have the habitat in some sense saturated 

 throughout? 



D. Jenkins: Yes. I think tliis is a correct interpretation. 



M. E. Solomon: If you are right about the existence of a large number 

 of wandering grouse, presumably this means that the density of grouse on 

 the ground may be regulated by some pattern of territorial behaviour? 



D. Jenkins: Yes. I beheve that territorial behaviour may be of real 

 significance. Cock grouse are first territorial in August/September, at a time 

 when the first big fall in population density occurs. At this stage there is an 

 initial high density of birds not behaving in a mutually antagonistic manner. 

 The males then take territory and are pugilistic, whereas the females still 

 flock. However the cocks are only territorial in the mornings and may feed 

 in flocks later in the day. This pattern persists through the autumn. At the 

 end of the winter the females also become territorial and become sexually 

 responsive. This change corresponds with the second drop in population. In 

 the early winter the females adjust their numbers to the males, going around 

 in pairs though not constant ones: numerically the population is paired but 

 the individual alliances change. Thus in the autumn males alone are territorial 

 and the females fit in wherever they happen to be, while in the spring both 

 are territorial and so bring about a stricter population Hmitation and the 

 second fall in numbers. 



A. Watson: In the spring, also, the birds are more consistently terri- 

 torial — in the afternoon as well as the morning: the whole behaviour is 

 intensified. 



G. SuRTEES: We seem to have had two examples today of migration 

 into and out of a sample area leading to a stable population — in both 

 grouse and roe-deer. In an exploited area we must postulate either an entry 

 of animals or an exit. If entry, the population wiU be maintained with stable 

 numbers, but if the habitats throughout the range of the species are unequal 



