Collins New Naturalist Library. R. Murton K.

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      FIG. 3a. Percentage change in numbers of grey partridges between successive breeding seasons (abscissa) related to the corresponding autumn ratio of number of juveniles per adult (vertical scale). The correlation coefficient is statistically highly significant with r₁₁ = 0.822.

      3b. Percentage change in numbers of red-legged partridges between successive breeding seasons (abscissa), related to the corresponding autumn ratio of number of juveniles per adult (vertical scale). The correlation coefficient is not significant with r₁₁ = 0.367. (Data derived from Table 1, from Middleton & Huband 1966).

      Those mortality factors which, like chick loss, affect the size of the actual population, have been termed ‘key factors’ because they provide the key to predicting future population size, and are responsible for the year-to-year fluctuations in numbers. Perrins’s work on the great tit and my own studies of the wood-pigeon had earlier demonstrated that, in these two species, the major factor influencing changes in numbers from one year to the next is the survival rate of young after the breeding season, not the reproductive output itself nor the adult loss. Juvenile survival has been proved to depend on the food supply in the case of the wood-pigeon, and there is good reason to believe that it is also involved in the case of the great tit. However, for both these species, and unlike the Hampshire partridges, juvenile mortality is not seen to be density-dependent (at least not with the data so far available) and so some other cause of death could be responsible for regulating the populations, in the strict sense of the term. Lack suspected, for the great tit, that winter losses in relation to food supply may provide the critical density-dependent mortality, but emphasised the difficulty of proving this point. The problem is that food stocks and bird numbers vary considerably in different years. It may happen that if food stocks are high, bird numbers can be already low or high so that in neither case is any compensatory mortality required. It is difficult to isolate such effects, because in field studies it is not feasible to examine a variable number of the animals in relation to the same food supply each year. Failure to obtain the required data does not in this case invalidate the theory. It should be mentioned that a population will be regulated even though most of the deaths which occur are not related to density, provided that a small density-related adjustment is eventually applied. For instance, 6o%-8o% of losses could be suffered quite at random provided that after these had occurred there was a small loss, say in the order of under 10%, which was related to density. Indeed, this is the more usual situation.

      On the Norfolk estate under discussion, red-legged partridges have increased over roughly the same period that the grey have declined (see Fig. 26 and Chapter 7, p. 176). It is interesting that, in contrast to the case of the grey partridge, the autumn ratio of young to old has not been the factor which determines the subsequent spring population of the red-legged partridge (Fig. 3b). There were some years when more red-legs were shot in the autumn and winter on the estate than were actually known to be there at the beginning of the shooting season, and numbers sometimes were higher in spring than in the autumn. Clearly, something has been happening which has enabled immigrant red-legs to move into the area from surrounding farms or marginal land (see here). This suggestion highlights another facet of population control which must be considered, namely the role of immigration and emigration. In the case of the wood-pigeon, juvenile birds which are surplus to the carrying capacity of the area in the autumn do not necessarily die but may emigrate to other areas – to marginal habitats or even to France. A proportion survive to return in the spring. Although winter numbers may be directly related to the food supply – for the death-rate depends on how many surplus birds exist in relation to this food supply (in a poor year for food there is not necessarily any mortality if the population is already in balance with food stocks) – the number in spring will depend not only on local survival but also on how many of the emigrants survive to return. These complications do not invalidate the contention that in any area at the worst season numbers are regulated by the food supply (for the rest of the year there may be more food than birds to eat it, especially if breeding cannot be accomplished quickly enough), but they add to the difficulty of demonstrating clear-cut relationships, and are sometimes introduced in ill-conceived arguments as evidence against the theory.

      Fig. 4 shows the autumn population, in decreasing order of size, of the grey partridge on the Norfolk estate against the percentage of birds dying in winter, either as a result of shooting or through natural causes. Similar data for the red-leg are also detailed, these being plotted against the appropriate years for grey and not in their own size order. The figure demonstrates in the case of the grey partridge that mortality due to shooting and other factors combined is positively correlated with the size of the autumn population, that is, it is density-dependent. The percentage of birds shot is even more strongly correlated with the numbers available for shooting and is also correlated with the total mortality. In contrast, natural mortality is not correlated with autumn numbers nor with shooting loss. That death by shooting should be adjusted to the autumn population is to be expected, since the people involved determine their activities according to the prospects; in years when autumn numbers are small, with a low young to old ratio, shooting is voluntarily abolished or curtailed. This attitude is mistaken for the simple reason that, in spite of shooting, additional animals have in any case to be lost to ensure stability in the spring, and if none are shot more disappear through other causes. This is well shown in Fig. 4 for the year 1954 when little or no shooting occurred and the level of natural mortality was raised. As a result the death-rate was at virtually the same level as it was in 1952 and 1955, when extensive shooting took place. Although enough animals are shot to ensure a density-dependent controlling effect on the winter population, this is only so because shooting obscures other causes of death. Because shooting may actually account for deaths it is not the reason why these occur: they would also occur in the absence of shooting. Jenkins reached this same conclusion for partridges he studied on Lord Rank’s estate at Micheldever in Hampshire, and he and his colleagues (Jenkins, Watson and Miller) have more recently found that the same applies in the case of the grouse – the details are shown graphically in Fig. 5. The practical conclusion to be drawn from these studies is that people could as a rule enjoy more intensive shooting without detriment to game stocks – though this conclusion caused scepticism among many shooting men. It is ironical, therefore, that my colleagues and I found exactly the same principle to apply to a pest bird, the wood-pigeon. The number of these shot during organised battues in February was always less than the number which had to be lost to bring about stability in relation to clover stocks. We concluded that winter shooting was not controlling the population, nor in the circumstances did it reduce crop damage. Again this conclusion caused scepticism among many shooting men who could not appreciate that causes of death are compensatory, not additive. Only if more birds are shot than will be lost in any case can shooting become a controlling factor, in the sense that it will reduce numbers to lower levels in the next season.