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ОглавлениеA THOROUGH insight into the relationships between birds and humans demands some understanding of population ecology, a knowledge of how animal numbers fluctuate and change through births and deaths and of the factors which determine these processes. Populations have dynamic properties and these cannot be neglected by people concerned with wild life management, whether as game preservers, conservationists, pest controllers or farmers. This chapter attempts to set out some of the basic principles with pertinent examples, but these same principles will emerge in later sections in various guises. Our knowledge of the subject was first collated and clearly enumerated by Dr D. Lack (1954) in a stimulating book The Natural Regulation of Animal Numbers since when more field studies have been made by various workers, which Lack (1966) has summarised in his Population Studies of Birds. Both books should be consulted by the reader who is really interested in this subject. In this account I have drawn on examples which, wherever possible, have direct relevance to economic ornithology.
Farmers tend to the pessimistic belief that all problem birds become more abundant every year. Yet careful counts of most such species living in stable environments usually show there to be no clear tendency towards a steady increase, or even decrease, though numbers may fluctuate from year to year. For instance, many farmers believe that the wood-pigeon has increased drastically during this century to become more of a pest, though in conditions of stable agriculture this is unlikely to be true. In fact, there is evidence of a decline since the early 1960s associated with a reduction in the acreage of winter clover-leys and pastures. Certainly the species has moved into newly developed marginal land; places like the east Suffolk heathlands, which have been ploughed and claimed for agriculture since the Second World War, have been colonised by wood-pigeons. Fig. 1 gives some indication of how wood-pigeon numbers have varied on a Scottish estate near Dundee since 1887, and it is evident that fluctuations have occurred within narrow limits, with no evidence at all for a sustained rise or fall. In contrast, Fig. 1 also shows how the closely related stock dove has increased over the same period; this species first colonised in Scotland in 1866, reached Fife in 1878 and increased dramatically in Scotland in the next ten years. Every year each pair of wood-pigeons rears on average just over two young and it is evident that if all these survived to breed in their turn, population size would increase exponentially. That this does not normally occur indicates that some form of regulatory mechanism must be operating. Furthermore, this regulation must be density-dependent, that is, it must become proportionately more effective at high population densities and proportionately less effective at low ones. If the regulatory factor (s) operated without regard to density, it is evident that population size could fluctuate widely without reference to a particular level – to the constant mean represented by the dotted line in the figure.
FIG. 1. Annual number of wood-pigeons and stock doves shot on an estate near Dundee. The data refer to an area which remained virtually unchanged during the period under review, and nearly all the birds were shot by one man who maintained a reasonably constant shooting pressure. They, therefore, probably provide a fair index of the total population. The autumn of 1909 was a disastrous one for the harvest owing to gales and rain from August until October so that much corn remained uncut. This resulted in an influx of wood-pigeons and the appearance of stock doves in large numbers. (Data by courtesy of Dr J. Berry.)
FIG. 2. Logarithmic increase of the collared dove in Britain. (Data from Hudson 1965).
Sometimes bird numbers do in fact increase geometrically, as when a species moves into a previously untenanted region where there is scope for it to live; in biological terms where a vacant niche exists (see below). In this way the collared dove dispersed dramatically across Europe to reach Britain in 1952. It had spread to south-east Europe from northern India by the sixteenth century but had then remained static in its European outpost until 1930. It has subsequently spread north-west across Europe, reaching Jutland in 1950 and Britain in 1955; an expansion 1,000 miles across Europe in twenty-four years. Why this spread was so long delayed is not clear, though it is most feasible, as Mayr has suggested, that a genetical mutation occurred which suddenly rendered the species less restricted in its needs. (We can imagine a bird, though not necessarily the collared dove, to be restricted by a temperature tolerance which a single sudden genetic mutation could remedy.) Throughout Britain and Europe a vacant niche existed for a small dove living close to man; it is even possible that the decline in popularity of the dove-cote pigeon created this niche. Whatever the explanation, a high survival rate among collared doves has evidently been possible, and their potential capacity for geometric increase has been realised: see Fig. 2, which relates to Britain. The Syrian woodpecker may be on the brink of a similar explosion. It is considered to be a recently evolved species (post-glacial) which replaces the great spotted woodpecker in south-east Asia. It spread to Bulgaria to breed in 1890, to Hungary in 1949 and to Vienna in 1951. A significant feature of the bird’s ecology in Europe is its confinement to cultivated areas which do not suit the great spotted woodpecker. If it proves to be better adapted to this man-made niche we can anticipate a continuing advance across Europe. Agricultural development in the Balkans may well bring other surprising range extensions.
Accepting* that populations are controlled in a density-dependent manner the next question to consider is the nature of these regulatory processes. Fundamentally, either the birth-rate or death-rate must be the factor of change. Wynne-Edwards and his supporters have argued that the reproductive rate is of much importance and that those animals with a high expectation of survival, such as many seabirds, have evolved low reproductive rates and vice versa. They have also claimed that a host of conventional behaviours have evolved as a means of regulating numbers. For instance, Wynne-Edwards regards the eating of eggs and young, practised by many raptors and also storks, as a device to limit their reproductive output; deferred maturity (gulls do not breed until three or four years old), territory formation, and various other behaviours are similarly regarded in this light. These views form part of his more general thesis that animal numbers in undisturbed habitats are at an optimum density and that maintaining this optimum has selective advantages. Special cases of this theme have attracted various supporters. E. M. Nicholson (1955) is one and he ascribes population control to density-dependent movements, rather than to mortality, while Lidiker (1962) goes further in seeing emigration as a mechanism enabling populations to have densities below the optimum carrying capacity of their habitat. Hence, Wynne-Edwards regards behaviours such as those listed above as homeostatic (self-regulatory) mechanisms, being induced by the population rather than by the environment.
Wynne-Edwards also drew parallels between bird and human populations, claiming that infanticide, taboos and other methods of reproductive restraint practised by so-called primitive societies were extensions of these same deep-rooted animal behaviours. In so doing he resuscitated some very early ideas of Carr-Saunders (published in 1922) which the well-known demographer did not subsequently repeat. Indeed, Mary Douglas, in criticising the idea of an optimum population, points out that there are many examples of human under-population. Considering certain primitive societies in detail, she makes it clear that they represent highly evolved and complex groups about which generalisations are meaningless. For instance, the Netsilik Eskimos live in a harsh environment where males suffer a very heavy mortality in hunting, and female infanticide is practised primarily to maintain an even sex balance. The Ndembu, a Lunda tribe in Zambia, grow cassava as their staple crop and live at a density of 3–5 people per square mile, whereas cassava could support a density of 18 people per square mile. The reason for the discrepancy is that the tribe passionately love hunting and move their villages to where game is available so that they never reach a stage where they are up against the ceiling imposed by their basic resources. As Douglas says: ‘they live for the oysters and champagne of life not the bread and butter.’ On the other hand, the Rendille are a tribe of camel herders in Kenya and live on the meat and milk of their sheep and camel herds. An optimum number of people is needed to maintain the camel herd, and when smallpox reduced man-power, stock had to be lost. In more normal conditions a balance of man-power is achieved by emigration, monogamy (herds are not divided but go only to the eldest son), while a measure of infanticide is practised (all boys born on Wednesdays).
In general terms, homeostatic population control in human groups depends on limited social advantages (the enjoyment of hunting by the Ndembu; education, motor cars and social prestige in western Europe) and not on any relationship to resources per se, as is the case with infra human species. Douglas could well be right in not attributing the increase in the Irish population between 1780–1840 to the adoption of potatoes as a staple diet, but rather to the ruination of Irish society by penal laws and English trade tariffs. Similarly she argues that the miseries of enclosures and the Poor Laws resulted in the population explosion which led to the Industrial Revolution, rather than that increased resources stimulated an increase in population. If true, this is quite different from the way in which animal populations are determined.
Animal populations cannot be directly compared even with the supposedly most primitive of human ones. Moreover, unequipped with any cultural tradition and means of communication, some special mechanism would have to be found to account for what amounts to altruistic behaviour. The Darwinian viewpoint is that if two animals (of the same or different species) are in competition, the one able to maintain the highest reproductive output must succeed at the expense of the other, all else being equal. This is the meaning of natural selection, which amounts to individual selection. What then would prevent the genes of parents which practise restraint from being at first swamped and then lost in competition with less socially inclined individuals? Wynne-Edwards surmounted this problem of explaining how individuals could acquire genes which cause them to behave in socially advantageous ways, sometimes at their own expense, by invoking the concept of group selection. In other words, in many situations survival of the group is more important than survival of the individual. Without entering into detail it must be said that the genetical basis for group selection is very restricted, and it can only be shown to be feasible in small isolated populations (birds do not satisfy the requirements of isolation as envisaged by geneticists) or in those, such as the social insects, where numerous genetically identical individuals are produced from one female, for which the term kin-selection is more appropriate. It seems to me that all the examples given by Wynne-Edwards can be answered (or will be when knowledge accumulates) more satisfactorily by individual selection and that to introduce group selection is unnecessary. This applies to two situations which I have studied closely – the peck order in birds and stress disease. I myself, therefore, reject the concept of an optimum population in this sense, together with the view that animals impose their own control over population increase. I have discussed the subject at some length because it defines my approach to the subject of applied ornithology. So far as reproduction is concerned I fully support Lack’s thesis that the reproductive rate has evolved as the highest possible; selective disadvantages follow from the production of more or fewer young than the optimum number. Disadvantages which accrue from overproduction include reduced survival chances for the offspring because they are undernourished, or impairment of the parents’ health; underproduction leads to a failure in intra-specific competition with 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 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.
FIG. 4. Relationship between the autumn population of the grey partridge (top hatched histogram) and red-legged partridge (third histogram from top, hatched) with the percentage of these populations lost between autumn and the following spring represented in the histograms below each. The percentage of birds lost is given by the open columns, and the percentage of these birds which were shot by the solid black. Loss due to other causes, is therefore, the difference between black and open parts of the columns.
[For the grey partridge the total mortality in winter is correlated with the autumn population being heavier in years of higher autumn numbers r11 =0.827. The percentage shot each winter is correlated with the autumn population r11 =0.909. The percentage shot is not correlated with the number lost for other reasons r11 = —0.420.
For the red-legged partridge the total mortality in winter is less correlated with the autumn population of red-legs, r11 =0.583, and more correlated with the autumn population of grey birds r11 =0.747. The number of red-legs shot bears no relation to the autumn population of red-legs but is related to the autumn population of the grey, r11 =0.789.]
The reason for some of the apparently anomalous results with the red-leg in which a higher proportion of birds was sometimes shot than was present in autumn is that the birds were moving into the area. Thus in 1952 the autumn population was 214, 259 birds were shot yet the spring population was 120, i.e. 121% of the autumn population was shot, there was a 77% loss not due to shooting (theoretically the difference between the total loss from autumn to spring=44%; with the loss due to shooting subtracted = — 77%, and this has to be shown by drawing the column below the base line. (Data derived from Table 1, from Middleton & Huband 1966).
The number of red-legged partridges shot on the Norfolk estate has borne only a slight correlation with the autumn numbers (Fig. 4). As the red-leg was much rarer than the grey, any decision on the numbers to be shot was made relative to the latter species, or more accurately to the total numbers of both species, rather than to the autumn population of the red-leg. For this reason the percentage of red-legs shot was fairly strongly correlated with autumn numbers of grey birds, while the total winter mortality (shooting plus natural) was slightly less strongly correlated. This illustrates how the amount of predation (in this case shooting) suffered by a species may be determined by the availability of similar prey. In such circumstances, the situation may arise where undesirably large numbers are lost by accident, even resulting in the extinction of a species.
Still considering Fig. 4, it is possible to imagine that this represented a species where the shooting had been done for pest control and not for sport, and that it was supported by a bounty. It becomes evident that a bonus scheme, unless it actually results in more animals being killed than would die in any case, would in this case prove a complete waste of money as a means of controlling numbers. It could only be justified if the animals concerned were killed before they caused damage. While the undesirability of a direct subsidy is fairly evident, there are often cases where grant-aid is paid in an indirect manner which obscures its futility. Variations in kill dependent on population size, and hence variations in subsidy, would be anticipated in different seasons – yet it usually happens that the amount claimed for a subsidy stays fairly constant. This is so in the case of the amount paid for wood-pigeon shooting. This suggests that only an arbitrary cull is being achieved; arbitrary in the sense that people now do roughly the same amount of shooting each year, and claim a fairly constant and acceptable level of support. It is extremely unlikely that this reflects realistically the variable level of crop damage caused by pigeons.
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 noticeable in northern areas; in central England, the decadal mean temperature of the summer months had risen about 2° C between 1900 and 1950, while a similar increase occurred in Finland and elsewhere. The temperature increases were particularly noticeable further north and at first were limited to the winter months (a 9° C increase in Spitsbergen), causing arctic ice to melt and polar seas to become warmer, but leading by the 1880s to increased temperatures during the northern springs, and to a summer increase in the 1920s.
Since about 1950 there has been a reverse trend in weather conditions which can be expected to bring about another southern displacement of the avi-fauna. Another complication is that the increase in mean temperatures has, in the maritime countries like Britain, resulted in distinctly wetter and cloudier summers. This is doubtless the reason why certain birds which depend on large flying insects have declined in recent years. Red-backed shrikes still produce more than enough offspring to ensure their increase, given the right conditions, but the bird has markedly declined in Britain and other parts of north-west Europe. Destruction of their habitat has sometimes been held to explain the decrease but this is by no means always the case, many areas now untenanted by shrikes appear to be unchanged, certainly in several Suffolk and Surrey localities that I know personally. Peakall, who has documented the decline, believes that a changing climate provides the explanation as flying insects become scarce on grey cloudy days. This was brought home to me very clearly when photographing red-backed shrikes on a Surrey heath in 1967 (see Pl. 1). During a six-hour session in a fully accepted hide when it was dull and overcast, the adults were clearly finding it difficult to get food. Even though they had large hungry young, each parent was visiting the nest about once every hour, bringing mostly ground beetles, until eventually they found a nest of young birds, and for a while flew back and forth with the nestlings. The following day was bright and sunny and the adults were feeding the young every five minutes or so, bringing dragonflies, butterflies, bees and lizards – in fact all the creatures which depend on sunshine to become active.
Other birds which depend on large flying insects, and which are at the edge of their distribution in north-west Europe and Britain, seem to be suffering a similar south-east contraction in range. Monk has shown that in 1850 the wryneck was very common in south-east England and the midlands and was scarce though regular north to the borders and west in Wales. By 1954, only 365 pairs could be accounted for and the total dropped to about 205 in 1958, since when it has fallen further. Moreover, well over three-quarters of the British breeding population is now confined to Kent. According to a survey carried out in 1957 and 1958 by Stafford, the nightjar is much more widely distributed in Britain, and although most common in southern England, it is frequent in the north of England and Wales though only irregular and local in Scotland. As it feeds on night-flying insects it has presumably been less adversely affected by cloudy summers. Even so, the survey showed that a decline has occurred during this century – though increased disturbance may be an important factor with this large and quiet-seeking species.
The stonechat, a bird of gorse commons, seems to have declined for a different reason. It feeds on large insects but catches them on fairly open ground, using a convenient bush as a vantage point. The ground and low herbage dwelling invertebrates on which it feeds can be found even in winter, so it remains a resident, like that exceptional Sylvid warbler, the Dartford warbler, which locally shares a very similar habitat. Magee, in a study of the stonechat in 1961, obtained breeding records from only twenty-three counties in England and seven in Wales, mostly only small numbers being involved. Only nine English, four Scottish and three Welsh counties had more than twenty pairs, whereas at the turn of the century the species bred in every English county. In 1961, 264 pairs were recorded in Pembrokeshire, 262 in Hampshire, 150 in Glamorgan, 104 in Cornwall, 96 in Devon and 51 in Dorset, after which Surrey with 39 pairs had the next highest English or Welsh county total. Magee pointed out that counties like Cornwall and Pembrokeshire have long stretches of coastline and still have extensive bracken and gorse headlands providing suitable conditions for the species. Otherwise poor gorse-covered commons or alternatively heath moor with numerous bushes have been largely lost as a habitat in Britain; the only extensive areas are to be found in those counties where tolerably high numbers still occur. Another complication is that severe winters hit stonechats very hard and it may take several years for numbers to recover. In this connection the coastal counties are probably less seriously affected, and Magee gives evidence that following any hard winter, recolonisation is usually first noticed in coastal areas from which birds then spread to inland habitats. In recent decades the predominantly mild winters between 1917 and 1939 allowed a fairly extensive re-occupation of inland habitats after the drastic reductions of the 1916–17 hard winter (41 consecutive night frosts at Hampstead) only to be followed by extensive reductions again with the hard winters of 1939–40 and subsequently. Hard winters are not regulatory factors in the accepted sense. Blanket snow cover or extended frosts affect wide areas irrespective of the number of animals present which must all suffer in a density-independent manner. A measure of density-dependence may be imposed if the animals are able to compete for restricted pockets of food; but this is a special case.
In south-west Europe, Cetti’s warbler is one of a small group of resident warblers which are similarly sensitive to hard winters and this applies to the resident Dartford warbler which is on the edge of its range in southern Britain. Cetti’s warbler spread north in France during the 1940s and 1950s with the period of mild winters and these also enabled a large proportion of Dartford warblers to survive in winter and then spread to other suitable habitats; a similar population increase and irruption to new areas occurred in the bearded tit for the same reason. The Dartford warbler formerly extended from Suffolk and Kent to Cornwall, but fragmentation of suitable heathland at the turn of the century has virtually restricted it to the New Forest, this being the only large enough area of suitable habitat, within its range, which can provide stability. Tubbs has demonstrated how during open winters the population can build up to occupy other heathland in Surrey and north Hampshire, where too small a population exists to withstand bad winters and where it has been exterminated two or three times since the 1940s – the winter of 1947 proving particularly disastrous. Tubbs has rightly emphasised the importance of maintaining the New Forest as a reservoir for the species. Here its numbers increased from around 80 pairs in 1955 to 382 pairs in 1961. Then came the snow of 1962 which exterminated the species in Surrey (the birds were trapped while roosting in tall heather by an overnight blanketing fall of snow) and left only 60 pairs in Hampshire. The following hard winter of 1962–3 virtually exterminated even the New Forest population, as well as causing a big retreat to the south of Cetti’s warbler in France.
Apart from changes in range caused by weather, several species have recently increased in response to addition to available habitats, which are often man-made. In inland Eurasia the little ringed plover replaces the ringed plover and nests on the sand and pebbly shores of slow-moving rivers or inland lakes, predominantly near fresh water. A pair nested at the Tring reservoirs, Hertfordshire, in 1938-the first nesting record in Britain. No further nests were found until 1944, when two pairs nested in another part of Hertfordshire, and one pair in Middlesex; the increase since then is shown in Fig. 6. By 1962 there were approximately 157 in Britain, nearly two-thirds being concentrated in the counties south of the Welland-Severn line and none extending further north than Yorkshire or west of Gloucestershire or Cheshire. Parrinder, who has documented these changes, points out that gravel pits provide over three-quarters of the nesting sites of little ringed plovers; sewage farms, reservoirs, brick pits and such places comprise the remainder. Most gravel-pits are centred in the areas where the species already occurs, except that a surplus of potential sites appears to exist in Lancashire, Northumberland and Durham and parts of Wales and these sites may next be occupied. Parrinder emphasises how gravel and sand production for building and road construction have increased with the post-war building boom and, as much of the material is derived from new workings, these have also increased. There can be little doubt that the creation of a new environment has favoured the species. It is interesting that our ringed plover has not been able to colonise these places, particularly as a local inland breeding population occurs in the East Anglian Breckland and the bird also breeds inland on some of the east Suffolk heathlands and in parts of northeast Scotland, for example in the Abernethy Forest.
FIG. 6. Increase in number of pairs of little ringed plovers summering in Britain. (Data from Parrinder 1964).
FIG. 7. Changing status of black redstart in Britain showing increase in number of territory holding males during the 1939–45 war with a decline following the final clear-up of war damaged sites after 1950. The solid line gives figures for the whole of Great Britain, while the dotted line is the contribution made by the City of London and Dover combined. (Data from Fitter 1965).
The black redstart was originally a bird of the warm montane regions of the southern Palaearctic, like the rock thrush and crag martin, but spread northwards after the last glaciation. It has adapted itself to a man-made environment, using buildings for nesting sites in lieu of cliff faces, and in parts of Germany it replaces the robin as the familiar follower of man. Its northward spread was still in progress across Germany during the last hundred years, and it only reached Jutland in the second half of the nineteenth century, and Scandinavia in the early 1940s. There is no record of it in Britain before 1819, though it became a regular passage migrant on the south and east coasts during the middle of the nineteenth century. Sporadic nesting attempts followed, in Durham in 1845, Sussex in 1909 and then a slow build-up from 1923 onwards. Three pairs nested at the Palace of Engineering in Wembley, Middlesex from 1926–41 but altogether less than half a dozen pairs were breeding in Britain up to the mid 1930s after which a big increase occurred, detailed in Fig. 7. The ability of the species to establish itself in this way has followed the sudden availability of nesting sites and feeding grounds, in the form of war-time bombed sites, particularly in London and Dover. Rebuilding and the post-war clean-up account for the subsequent decline in numbers.
There are also examples of bird numbers reduced through direct persecution, especially when the victim is fairly rare. The great crested grebe was certainly widely distributed in suitable places in Britain in the early nineteenth century, but by the middle years of the century a big demand arose for its breast feathers to make ‘grebe furs’ for a fashionable home market, and the slaughter began in 1857. By 1860, the species was reduced to 42 known pairs and was only saved by the sanctuary afforded by private estates, while further help came with the Bird Protection Acts of 1870–1880. Nevertheless, some increase was under way by 1880, before protection could have been very effective, and Harrison and Hollom (1932), who record these early changes, consider that human persecution came at the start of a period of long-term cyclical increase. By 1931, there were around 1,150 breeding pairs (with non-breeders, about 2,650 adults) in England and Wales and about another 80 pairs in Scotland. A sample census by Hollom (1959) showed that about the same number of adult grebes existed in Britain in the 1940s, but that an increase then began. When Prestt and Mills undertook a census in 1965 there were approximately 4,500 adults in Britain. This increase seems to have been favoured by man’s activities in creating numerous new reservoirs and gravel pits, just as the little ringed plover benefited. The 70% increase of the population in about twenty years can be compared with an increase of 84% in sand and gravel production between 1948 and 1957. That an increase followed the creation of new habitats also indicates that saturation had previously been attained and that the bird was regulated in the sense already discussed.
For a species to extend its range and take advantage of newly developed habitats it would be helpful for it to possess some kind of exploratory behaviour rather than rely on chance movements. It is becoming clear that immediately after breeding, many species, which normally migrate south, first indulge in northerly flights. The large-scale ringing of sand martins has shown that birds breeding at a colony in the south of England may move north to have their second brood, and juveniles marked in southern England have been found again in roosts in the north in the same season. Wood-pigeons also display northerly flights in September and October, before adopting a southerly orientation later in the autumn. Collared doves ringed in Europe as nestlings have moved north to Britain in the same autumn, and numbers of serins have turned up in south-west England in recent autumns. These movements seem adaptive in that young individuals which are surplus to the needs of the area in which they are born will stand more chance of finding new places to settle if they first explore north. The same principle applies to those birds of southeast and east Europe which might be expected to move north-west or west. I suspect that this factor may account for big arrivals of redbreasted flycatchers, woodchat shrikes, barred warblers, melodious and icterine warblers – all predominantly juvenile – into Britain in September 1958 and in subsequent years. Williamson showed that red-breasted flycatchers and icterine warblers arrived in Britain in clear anticyclonic weather with light winds, and as both migrate south-east to Asia, their movement several hundreds of miles off-course is remarkable. The explanation that they drifted in with down winds seems unlikely, and instead I wonder whether the existence of anticyclonic weather facilitated a normal adaptation after breeding in the form of a deliberate dispersal north-west, a process possibly truncated in years of less favourable weather.
Man has done so much in a passive way to alter the avifauna of Europe that it seems reasonable to take active steps to reintroduce lost species. Any reservations that this would be unnatural, should be tempered by the thought that the environment we have created is in any case artificial. Probably more pleasure than harm has been derived from the reintroduction of the capercaillie. It would seem laudable to follow up a recent suggestion and attempt the reintroduction of the bustard to parts of the East Anglian Breckland, and to encourage black terns to stay and breed. It is quite a different matter to introduce alien species to a new country, especially without sound biological knowledge. In Britain, some of these introductions, red-legged partridge, various pheasants, little owl, Canada and Egyptian goose and Mandarin duck have on balance improved our bird-life, but the same could not be said of the introduction of the house-sparrow and starling to Australia and North America.
Perhaps a more interesting question to ask here is why the majority of introduced species are unsuccessful. This is part of the much bigger question of what factors determine faunal diversity and enable some habitats to support more species than others. The concept of a niche, which refers to the animal’s place in the biotic environment and its relations to food and predators, should now be widely appreciated. It is a fundamental tenet of ecology that no two species can occupy the same niche in any one habitat, because both cannot be equally well adapted. R. and J. MacArthur (1961) examined numerous habitats at different latitudes for their plant species composition and foliage profile and ‘species diversity’. They found this last to be a more useful measure than the actual number of species because their calculations allowed at a habitat containing 50 of species A and 50 of B to rank a higher diversity than one with 99 of A and 1 of B. (The latter tends to be the farmland situation, the former that of tropical forest.) It turned out that neither the variety of plant species nor the latitude affected the amount of species diversity which instead depended entirely on the variation in foliage height, probably because birds mostly respond to different configurations of vegetation in different layers. This means that habitats of the same structural profile have the same diversity of bird species. In any area, a bird might either feed on all food of a suitable size within a narrowly defined habitat or, alternatively, be selective of food but collect it throughout a wider range of habitats. In other words, birds could partition their food or their habitat. The former has occurred because feeding specialisation brings the greatest advantages and has been favoured by natural selection. Partitioning the habitat would necessitate birds moving from one suitable micro-habitat (say a species of tree) to another, and it would depend on the pattern of the total habitat how much time would be wasted in the process. But adaptation to a comparatively broad habitat structure, for instance, to arboreal or ground feeding, must in turn impose physical limitations which restrict the diversity of feeding adaptations; in practice, bill size and shape is about all that can be much modified to suit the collection of different foods.
From the viewpoint of zoo-geography, the Palaearctic has existed as an entirety for sufficient time to ensure that most niches are filled by highly efficient species. Furthermore, man and birds have lived side by side since Neolithic times, so that the new habitats created by agriculture and man’s other activities have been occupied by the species best suited to them. The same is not true of Australia and New Zealand, which were cut off from the main centres of evolution at an earlier stage, one result being that primitive marsupial mammals were not replaced by the better adapted placental mammals. Birds are less insular, however, and the native avifauna of Australia seems to be the best fitted to occupy the niches available. Thus of at least 24 bird species deliberately introduced into Australia in the past, only 12 have become established. It is significant that only the blackbird has managed to invade native forest, the remainder existing in areas of recent agricultural development or urbanisation. But introduced birds like the feral pigeon, starling and house sparrow are better equipped to occupy the man-made niches than the native fauna, simply because these are species which have already been selected to occupy a man-made environment. New Zealand has an impoverished avifauna compared with Australia on which it has depended for colonisation, and the process is still incomplete. In consequence, fewer niches are saturated in New Zealand, so more exotic species have been successful. Of 130 species originally introduced, 24 have become established, although apart from the blackbird, chaffinch and redpoll which appear to be filling unexploited niches, most are again restricted to man-made habitats. Hawaii, which is even more isolated, and not saturated by a wide diversity of species, must have even more vacant niches, for, according to Elton (1958), of 94 birds introduced 53 have become established, some deep in the native forest.
According to Middleton, the European goldfinch has been successful in Australia and New Zealand only in the man-made agricultural areas, to which none of the native Australian Ploceid finches were adapted. In contrast, the European goldfinch has not been a successful bird in North America because it has virtually the same ecological requirements as the fitter endemic American goldfinch. Only one small colony of European goldfinches became established near New York, though these have since vanished when their habitat was destroyed for building purposes. Again the European house-sparrow has been highly successful in Australia, over roughly the same range as the goldfinch, whereas the introduced greenfinch is more restricted as it has rather more conservative ecotone requirements. It is interesting that this reflects a trend occurring today in Britain; the greenfinch is declining with the loss of hedgerows and woodland edges, while the goldfinch and linnet are increasing.
To return to New Zealand, it is noticeable that the birds which have become pests in agricultural areas, apart from being introduced species as one would expect from the comments above, present the same kinds of problems as they do in Britain. I am grateful to Dr P. C. Bull for allowing me to give details. As we shall see, skylarks (see here) are locally troublesome in Britain to young seedling crops such as lettuce. Near Hastings, N.Z., they and house sparrows have together been responsible for damaging asparagus and other seedlings. Both blackbird and song thrush and also the starling, resort to orchards in the dry season after breeding and cause considerable damage to all kinds of fruit, ripening pears, cherries and grapes. Redpolls do considerable damage to apricot blossom in their search for insects, and blossom searching is a habit which is increasing in Britain (see here). Locally in Britain, linnets peck out the seeds from strawberries (see here), while in N.Z. goldfinches do the same.
Various attempts were made to introduce the rook into N.Z. from 1762 onwards but only 35, liberated near Christchurch in 1873, seem to have thrived. The species occurs in five localities on the yellow-grey earths in the east of the county, generally where cereal growing occurs. Rooks at first increased very slowly but there was a rapid increase between 1935 and 1950, and a final levelling off with the density of birds in their favoured areas becoming virtually the same as that in Britain (around 16 nests per square mile). At Christ-church, the population increased from 1,000 birds in one rookery in 1925 to 7–10,000 in 1947 (13 rookeries), since when the numbers have remained roughly constant with 19 rookeries in use. Until 1926, the rookeries were in eucalypts, probably the favourite tree, but following a disease epidemic which killed these trees the birds changed to pines. Bull points out that the rate of increase of rooks has been slower than that of other introduced passerines and attributes this to their gregarious nesting habits and their need for group stimulation, and to an early shortage of suitable habitats. A feature of N.Z. rookeries is their very large size compared with British ones (rookeries of over 1,000 nests are quite common), and the traditional return to the same nesting sites may partly explain the slowness of expansion. (The large size of rookeries, and difficulties in getting sufficient food locally may also explain why the birds seem to lay, on average, smaller clutches in N.Z.; 3.4 eggs against 4 + in Britain, (see here), though more data are needed to establish the point.) Frequently, only when man actively disturbed these large rookeries did they become fragmented in surrounding areas, often with a rapid increase in total bird numbers in the district. As in Britain, rooks uproot seedling peas and corn, take ripening peas (and pumpkins) and maize and are also partial to walnuts.
The number of closely related birds which can live in the same habitat without competing for food depends to a large measure on the degree of stability within the environment. Marked fluctuations occur on English farmland, not only because of the changing seasons, but also because ploughing, harvesting and other farm operations impose drastic changes. As a result, the farmland birds occupying the various niches available for ground-feeders show a wide character displacement; we find a plover, three passerines (rook, starling, lark), a partridge and a pigeon, other species being only transient visitors, or primarily dependent on other habitats. No bird can afford to be too conservative in its niche requirements in a fluctuating environment, while the need for each species to show more tolerance reduces the number of ecologically isolated forms. Therefore, we should expect modern farm mechanisation, which enables whole farms to be ploughed within a fortnight, to be detrimental to bird life compared with the old methods which ensured some degree of stability by leaving land fallow and by transforming stubbles into bare ground more gradually. Klopfer and MacArthur (1960) have similarly emphasised that the major factor accounting for a decrease in the number of species away from the tropics, while the number of individuals of each species increases, does not result from a decrease in habitat complexity, but to a decrease in the similarity of coexisting species. The principle can obviously be extended to any situation where man simplifies the environment.
An important feature of complex ecological communities is that interactions between members damp out oscillations in the numbers of any one species (see here) and so help to introduce a high degree of stability and energy utilisation. For one thing, available food is more fully exploited, which is not the case in arctic environments for instance, where considerable seasonal changes occur. Hence, the amount of energy needed to maintain a stable community is less than that required for an unstable one. Man’s activities have tended to reduce complexity and introduce monotony, through monocultures of crops, uniform stands of trees, or rows of similar houses. In consequence, the animals inhabiting these environments usually fluctuate much more than those of more complex ecosystems, often to the extent of becoming pests (see here). One feature of stabilisation is that natural selection can favour anticipatory functions – for example, the breeding season of northern birds has become approximately geared to seasonal daylight changes – in unstable environments opportunism must set more of a premium. Because more energy goes into maintaining fluctuations in simple ecosystems, often short-term fluctuations, these systems offer more scope for rational exploitation, giving more production per unit of biomass. In general, pestiferous birds and game species can be cropped very intensively, but the corollary also applies in that the energy needed to counteract fluctuations, the efforts of pest control, must often be so considerable as to be impracticable. On the other hand, mature and complex ecosystems can be disturbed relatively easily. In the Eltonian food chain the predators at each level become rarer and larger, because the energy passing from link to link is only in the region of 10%-20%. This not only sets limits on the number of links in a food chain (five seems to be the maximum) and rules out the possibility of a super-predator but makes the top predators particularly vulnerable to small but cumulative changes in the food chain. The loss and increased rarity of so many of the birds of prey depends not so much on persecution, but on the reduced complexity of the environment through human ‘progress’. Clearly our future policies should not concentrate too much on bird protection per se, but rather on the creation and maintenance of as much diversified habitat as possible.
The long-term or ultimate value to a species in settling in an appropriate habitat will depend on the bird’s ability to find suitable food and produce surviving progeny, and this ability will be conditioned by the structural and behavioural adaptations of the species. The immediate or proximate factors which determine how a bird chooses an appropriate habitat are unlikely to involve these same factors. Instead, natural selection has enabled each species to respond to immediate signals, which can be reliably taken as indicators that other more basic needs will be satisfied. In this way a bird which lives in oak woodland might respond to the configuration of an oak tree, because natural selection will favour this appropriate response provided it leads the bird to find in the oak woods all the various foods which are appropriate to its needs and feeding adaptations. Natural selection can favour the emergence of appropriate proximate responses which are anticipatory.
In practice, it is generally agreed that birds respond to a range of releasing stimuli which combine to provide the best cues. A meadow pipit may respond innately to open country, thereafter to specific elements of the habitat, such as the height of grass, the presence of song posts and nest sites. These considerations are important when man radically alters the habitat, without necessarily altering its food value. As various features of the environment combine to produce a response, they need not always be present in the same proportions, and some may even be absent, for a response still to occur. Species vary in the capacity to respond when some stimuli are absent. Within any area intraspecific competition ensures that the most favourable sites are filled at the start, after which less complete habitats can be occupied. At low population densities, when, for example, a species is at the edge of its range, only the best habitats are occupied (stenotopy) whereas when population explosions occur, marginal areas are also utilised (eurytopy). This is well illustrated by the wide range of nesting sites accepted by the various gulls which have undergone a spectacular population explosion in Britain – nesting colonies occur on rocky and sandy sea shores, estuarine and freshwater marshes, inland lakes, and on moors and fells.
It becomes clear how an originally montane bird like the house martin should come to accept the sides of houses for its nest site instead of cliffs. Similarly the absence of a species from what appears to be a suitable habitat may be attributable to the absence of some apparently trivial factor which must be satisfied. Already in south Europe it is known that the presence of electric and telephone pylons and cables in otherwise open country facilitates colonisation by species like the collared dove and various shrikes.
The psychological response of the reed bunting to a limited range of habitat cues seems to have been the only reason for its past restriction to wetland habitats and its ecological isolation from the yellowhammer. But, as will emerge later, this segregation is no longer maintained and the invasion of yellowhammer habitats by the reed bunting is possibly the result of a genotypic change which has removed this psychological restriction. Another explanation is also tenable. Although most habitat recognition is innate, birds are able to reinforce or even modify, to a variable extent, these innate responses by learning processes. By this means, adults often return to traditional areas, even though these change drastically, whereas young birds breeding for the first time avoid entering habitats which do not release appropriate responses – either innate or acquired by imprinting in early life. For instance, Peitzmeier (1952) found that in a study area in Germany the curlew typically nested only in boggy areas and avoided surrounding cultivated land. When the marshes were drained and cultivated, the adults not only remained faithful to the area, but after learning its new characteristics, also spread to other tilled land which before they had avoided. It could be that this situation has applied in the case of the reed bunting. Peitzmeier attributes the further spread of curlews in arable environments to the imprinting of young which have been reared in these new habitats. Such processes explain why local populations of birds come to acquire unusual habitat associations – for example, the stone curlews which used to nest on the shingle of Dungeness and Norfolk beaches, or yellow wagtails which breed in fields of growing potatoes. Peitzmeier accounts for a remarkable and fairly sudden change in the nesting habitat of the mistle thrush in the same way – originally confined to continuous woodland dominated by conifers it began, in 1925, to nest in parkland-type habitat, and in small groups of deciduous trees in cultivated country. That very recent changes may have occurred in the habitat of an animal must always be remembered when trying to understand their adaptations – they may have evolved in conditions quite unlike those in which the birds are seen today.
