It is seen that most of the annual fluctuations which occur in the numbers of any bird depend primarily on juvenile rather than adult survival. Most adult birds die not through starvation but by accident – by predation, occasional disease, and pure accidents such as flying into a telegraph wire. In general, big birds are less prone to accident; they are less likely to be caught by a predator and so they tend to have lower death-rates, but there are many exceptions. Established adults must have already experienced a season of food shortage, which they have successfully survived in competition with other individuals. It is unlikely that they will suffer in subsequent years, unless a particularly lean season occurs. In other words, food shortage may only seriously affect an established adult in one year out of many (a hard winter is one example of this). Moreover, in many cases most of the adults which die by accident do so outside the season of normal food shortage; adult starlings, for example (see here), are at greater risk of death during the breeding season, when they are busily occupied with minding their young and are more often caught unawares by predators. If a population remains stable (as in Fig. 1) but produces a large excess of young, it follows that a large number of these must die. This juvenile mortality should be seen primarily as a consequence of the young birds’ competition with the adults, whose greater experience nearly always enables them to survive better than their inexperienced offspring. Indeed, the number of young which will survive depends on how many adults are lost to make room for them, the final adjustment occurring at the worst season of the year for whatever factor is limiting adult population size. This can be food without it appearing obvious. Thus, after breeding, there exists a big excess of young although there may still be enough food to support all individuals. Accidental deaths will occur throughout this time, but it is likely that young will be most severely affected through inexperience. Eventually, and it may be gradually or suddenly, the season of minimum food supplies will arrive. If by this time there are already too many adults, then virtually all the young will now be lost as well as a few adults. If some catastrophe has occurred and no adults are available, then larger numbers of young will survive to restore the former balance between total numbers and environmental resources. But these two extreme situations will occur only rarely. The above account is slightly over-simplified, as in reality adults themselves do suffer a little from competition with their young, and the process is not completely one-sided. Removal of juveniles increases survival prospects for the adults. Furthermore, it will be appreciated that several factors influencing bird numbers may act simultaneously, so that adjustments are continuous – this is why animal populations are called dynamic.
FIG. 5. The top of the columns represents the total number of grouse in different autumns on study areas in Scotland on low ground (left) or high ground (right). The number of these birds which were shot is indicated by the solid areas, the number lost through other causes by open columns and the number of grouse alive in the spring by hatching. The data show that more birds must die than are actually shot. On high ground in 1961 there were more grouse in spring and autumn, as a result of immigration. (Data from Jenkins, Watson & Miller 1963).
Our studies of the wood-pigeon provide an example of some of these processes in operation. In 1959 the post-breeding population comprised 171 birds per 100 acres with 1.3 juveniles to every adult. In 1963 there were only 101 birds per 100 acres and 1.5 juveniles to each adult. The clover food supply, at the worst time of the succeeding winters of 1960 and 1964 was near enough the same and so the population was reduced to 34 and 35 birds per 100 acres respectively. But in the 1959–60 season the competition needed to bring about balance (171 down to 34) was clearly much greater than in 1963–4 (101 down to 33). The effect on the juveniles was striking. By the February of 1960 there were only 0.1 young to every adult against 1.1 in 1964. Hence, in both years total numbers reached the same level by winter, but juveniles suffered a 96% loss in 1959–60 against 74% in 1963–4. Adult loss in the first year was 59%, and 58% in the second season. These figures do of course illustrate a density-dependent loss of young. The term mortality has not been used, because some birds were lost through the emigration of both young and old, though it amounts to mortality so far as the carrying capacity of the land was involved in mid-winter. The true annual death-rate of adults was lower than the figures quoted.
This example shows how the age structure of a population may be altered without any change in its ultimate size. The red grouse provides another illustration of this effect, achieved by deliberate killing. It has already been noted that grouse numbers in spring are unaffected by shooting. Yet Jenkins and his team found that over the autumn the death-rates of adults and young were equal (at around 70% per annum), in sharp contrast to all other birds so far studied. This was because so many birds were shot that deaths from natural causes were not fully obvious; presumably shooting is not selective of either age group. If enough animals (old and young combined) are shot natural competition between adults and juveniles can be reduced. In an unshot population of white-tailed ptarmigan in Canada (Choate 1963) the juveniles did suffer a much higher death-rate than the adults. Again, young wood-pigeons are easier to shoot than adults until mid-winter, by which time they seem to have learnt to avoid men with guns and from February onwards are no more easily shot than old birds. But they still do less well than adults when competing for food (and at breeding sites later on) and for this reason suffer a higher death-rate. Clearly, the amount of winter shooting can alter the age structure of the population without affecting final numbers in summer.
As mentioned above, before any artificial killing can result in a reduction of population size, the total number of animals killed must exceed the rate of deaths from natural causes. In addition, as increasing numbers are killed artificially, natural mortality factors cease to operate and must be replaced by artificial ones if compensatory changes are to be avoided. Inability to reach the necessary threshold means failure so far as artificial control is concerned. If numbers need to be kept down it is best, all else being equal, to defer artificial killing to the season when natural factors have taken their toll. If such population control cannot be achieved artificially and numbers cannot be held below a natural optimum, it is vital to show that any cropping does prevent damage; it is of course feasible that artificial killing will remove animals before they would normally die, and so protects crops. But every case has to be taken on its own merits, and examples of wise and foolish applications will be found in later chapters.
The level around which a population fluctuates may be subjected to long-term changes not resulting from the key factors so far considered. Blank et al. have demonstrated that the number of breeding grey partridges (which have shown evidence of a general decline during the present century) is determined by the nature of the habitat. The favoured sites for nesting are an incomplete hedge, that is, a group of separate bushes often patchily arranged on a grassy bank, or a wide grass track. Jenkins (1961) also showed that a lack of winter cover (cereal crops as distinct from tall grass provide little cover) results in the formation of larger territories, because the males can see rivals at greater distances and this causes the breeding population to be lower; under these circumstances surplus birds move on to marginal habitats. It is conceivable that changes in land usage and farming techniques have harmed the partridge by causing the population to fluctuate around a lower level.
The well-documented decline of the corncrake seems to have resulted from farm mechanisation, which has eliminated the old hand cutting of hay and enabled the harvest of silage to take place earlier, to the detriment of nesting corncrakes. Once generally distributed throughout the British Isles, the species disappeared from East Anglia before 1900, from east midland and southern England by about 1914, and from Wales, northern England and east Scotland by about 1939. It remains common only in Ireland, parts of western Scotland and the Hebrides, Orkneys and Shetland – areas where the old methods of hay production to a large extent remain unchanged.
Britain has experienced several periods of radically altered climate which must have considerably affected bird distribution, particularly of those species at the edge of their range in northern Europe. Since the mid-nineteenth century changes in air circulation have caused a warming of the atmosphere particularly