Experiments on Population Control by Territorial Behaviour in Red Grouse

Male red grouse (Lagopus lagopus scoticus (Lath.)) contest for territory each autumn, and some males are successful while others fail to secure territories. This paper describes experiments undertaken to test whether males which were not occupying territories could become territorial if the established territory owners were removed; that is to say, whether the number of breeding males was being limited simply by the territorial accommodation available or by some deficiency in the unsuccessful birds. Previous research consisted of counts of the grouse on 100-120 ha study areas on heather (Calluna vulgaris L. (Hull)) moorland in north-east Scotland, with more detailed studies of the behaviour of individually marked birds on smaller parts of these areas (Jenkins, Watson & Miller 1963, 1967). Territorial behaviour, courtship and pair formation are described by Watson & Jenkins (1964). The population studies showed that there were many more grouse in autumn, even after the grouse shooting was over, than in the following spring. The behaviour studies showed that grouse populations from October to May consisted of (a) cocks which courted hens and defended territories, plus hens paired with them (territorial birds), and (b) non-territorial birds which did not defend territories, show courtship, pair up, or breed. Classes (a) and (b) both included birds less than 1 year old (called 'young' in this paper) and older birds. On average, 52%0 of the August population later became non-territorial over the autumn and winter and died before the next April-May (Jenkins et al. 1967), whereas both young and old territorial grouse survived the winter well and bred next summer. Consequently we postulated (Jenkins et al. 1967) that possession of territory was essential for breeding, and that territorial behaviour in autumn greatly limited the size of the next spring's breeding stock. These hypotheses were open to the criticism that although territorial behaviour was associated with the population changes, it might not really be preventing the non-territorial grouse from taking territories. The crux was to find if they would take territories and breed when vacant ground was made available. If they did not, the hypotheses would be refuted. One might then explain the presence of non-territorial birds simply by suggesting that they were immature individuals, as in many other species where some individuals do not breed till 2 years old or more.