Collins New Naturalist Library. R. Murton K.

Чтение книги онлайн.

СКАЧАТЬ 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 СКАЧАТЬ