more fecund individuals.
As most birds produce a large excess of young, the total post-breeding population increases considerably, two-fold in the wood-pigeon, up to six-fold in the great tit, but by less than half in the fulmar. Various mortality factors, including disease, predation and starvation, now remove surplus individuals so that a balance with environmental resources is achieved, and a pattern of sharp fluctuations within each twelve-month period is superimposed on more subtle changes in the breeding population from one year to another. Lack has argued, and again I agree with his views, that the food supply is usually, but not invariably, the most important factor affecting these annual fluctuations in birds. While food could ultimately be the most important factor in all cases, other agencies, for instance predators, may hold numbers below the level which would be imposed by food shortage. The most important fact to appreciate from the viewpoint of economic ornithology is that causes of death are effective until the population is in balance with the environment, and in the absence of one such factor another will take its place. Conversely mortality factors are not usually additive – in the sense that two together do not decrease numbers more than one alone. The degree of stability seen in a population will, therefore, depend primarily on that of the environment and not on any essential characteristic of the population. By environment we not only mean the food supply but include all the other components, biotic and physical, which may interact to cause competition and mortality, directly or indirectly.
Blank, Southwood and Cross have recently shown in a neat but semi-mathematical way how the various causes of mortality contribute to the regulation of a partridge population on a Hampshire estate which was studied from 1949–59. I shall illustrate less elegantly certain aspects by using as an example another partridge population living on a Norfolk estate, for which details of annual fluctuations in numbers have been given by Middleton and Huband (Table 1). Following breeding, numbers increase over tenfold but there immediately follows a period during which it is mostly the young which are lost, so that from a post-breeding average of nearly ten chicks to each adult the ratio falls to only 1.5 per adult by August, most of this chick loss occurring in the first few weeks of life. Jenkins (1961) had earlier claimed that this heavy loss of young, which is particularly heavy in cool, wet and windy summer weather, was the main variable determining the number of partridges later available for shooting. This was confirmed by Blank et al., who emphasised chick loss as the major contributor towards the total mortality occurring each year in partridges, and as the most important determinant of the September ratio of old to young. They found that about half of the variation in total mortality was due to fluctuations in chick loss, but that about half this chick mortality was unrelated to the size of the population at hatching. This means that the survival of chicks was partly responsible for regulating the autumn population (in the sense that autumn numbers were largely governed by how many young survived). However, because this survival was only partly determined by population size there was a margin of production which caused autumn numbers to fluctuate partly independently of population density. Thus 35% of the year-to-year fluctuation in the September population resulted from variations in chick mortality. In further studies Southwood and Gross were able to show that 94% of the variation in breeding success (they used the ratio of old to young birds in September as a measure of breeding success, this also being a measure of chick survival as explained above) could be accounted for by variations in general insect abundance in cereal and forage crops in June. They measured insect abundance by the use of suction traps and showed that the insects sampled were in the main those eaten by partridges, judged by analysing the crop contents of chicks.
The number of partridges finally surviving to breed in March depends on mortality factors operating in the winter, shooting being the most important. Shooting is contrived to operate in a density-dependent manner with proportionately more birds being shot when numbers are high, but this is, of course, an artificial situation which masks the effect of natural regulatory agents. These last are likely to reside in the nature of the environment, the amount and type of cover which in turn influence territory size and may result in surplus birds emigrating. They are considered in greater detail below (see here) as they are involved as factors determining long-term changes in population size as distinct from annual fluctuations. To summarise, variations in chick survival dependent on arthropod food supplies are responsible for the marked ups and downs of partridge numbers from year to year. Density-related variations in chick survival, together with density-dependent winter losses, are responsible for keeping the spring breeding population within relatively narrow bounds from one year to another. Nevertheless, there has been a general long-term decline in this level which we shall consider below. It is to be noted that pre-hatching factors that influence the viability of eggs (the ability of the female to lay down yolk reserves could depend on spring food supplies and influence the viability of any eggs she laid), the hatching success of eggs, or any other cause of mortality, were not related to population size nor to variations in total mortality.
For the figures given in Table 1 it can be ascertained that the number of adults breeding in any one year was not at all related to the number breeding in the next year. In other words, a small breeding population could be followed by an increase or decrease in the following season and vice versa. But, as Fig 3a. shows, the percentage change in breeding population from one year to the next was positively correlated with the autumn ratio of young/old, this in turn depending on the survival of young during the summer. In Norfolk this post-breeding chick loss was not correlated with the size of postbreeding population. Thus for two quite separate populations of the grey partridge the major cause of changes in numbers from one breeding season to the next has been the death-rate of young in the summer months; when this has been low, breeding numbers have tended to increase. Annual differences in reproductive output have not contributed to the changes. In the Hampshire study the summer loss was dependent on the total partridge density, and this supplied the necessary regulation to keep fluctuations within relatively narrow limits. In addition, as we shall find, both populations experience density-dependent losses in winter, but the absence of any density related loss in summer among the Norfolk birds could be associated with the long-term decline this population is experiencing. (Fig. 26, see here.) However, this last suggestion needs corroboration.
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