Collins New Naturalist Library. R. Murton K.

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СКАЧАТЬ by a farmer ploughing a field, the gathering of swallows and martins to feed on the insects blown from a wood in strong wind, and the birds which gather round a locust swarm. Here, the number of prey eaten depends on the number of predators that chances to arrive on the scene, and is not a function of prey density. The scale of losses inflicted by birds on locust swarms seems usually slight. Around a small locust swarm in Eritrea, which covered about ten acres, Smith and Popov noted two or three hundred white storks, many great and lesser spotted eagles, and several hundred Steppe eagles, as well as smaller numbers of black kites, lanner falcons, marabou storks and other species. Shot and dissected storks each proved to have eaten up to 1,000 locusts. But most observers agree that this scale of predation has a negligible effect. More important is the suggestion (e.g. Vesey-Fitzgerald 1955) that birds may be useful in preventing a rapid build-up of locust numbers. Recent evidence from the Rukwa Valley indicates that this is not the case. First, because the preferred feeding habitat of the birds mostly concerned – white stork, cattle egret and little egret – is the short grass area of the lakeshore, whereas the locusts prefer and breed in the long grass associations covering much of the plains. Second, locusts are most abundant from March–June, when bird numbers are low, and decrease for other reasons in July when large numbers of immigrant birds arrive.

      When man preys upon wood-pigeons by shooting them on their return to their roosting woods, the number he kills increases slightly when the total population increases, but the percentage shot declines. Each man shoots at the passing flock but can potentially kill only two birds because he uses a double barrel 12-bore gun. More flocks pass when pigeon numbers are high, enabling a higher total of birds to be shot, but the flocks are also much larger and a smaller proportion of each can be killed. Hunting methods impose a limit on a man’s kill, and the only way to achieve a higher rate of predation would be for more men to shoot or for each man to use a faster firing, or otherwise more efficient gun. The first possibility is limited by social considerations; the number of men interested in shooting, either for sport or for monetary gain, is restricted because today there are so many more outlets for pastoral relaxation and the financial reward for shooting pigeons is low. The battue shoots did not usually begin until the end of the pheasant shooting season, in late January and early February, because only then were gameowners prepared to let pigeon shooters wander over the estates. They ended in early March when shooting at dusk on the longer days interfered with the other attractions of evening, the village dance or local hostel. The alternative of using a more lethal weapon would be opposed to the arbitrary code of sportsmanship current in Britain today, a code which imposes operose conditions on the ways animals can be ‘taken’ – a euphemism for ‘killed’. In days when cumbrous muzzle-loading guns prevailed, shooting birds sitting on the ground was acceptable, but fashion changed to ‘shooting flying’ during the eighteenth century as guns improved, and today shooting a sitting bird is unthinkable. Today a similar pretentious scorn is poured upon the American repeater, just as it was upon the double barrelled 12-bore when it first appeared. As Markland (1727) says in Pteryptegia or The Art of Shooting Flying:

      he who dares by different means destroy

      Than nature meant, offends ’gainst Nature’s law.

      A viewpoint all right in sport but with no place in serious pest control.

      Sometimes predators take all, or at least a large proportion of, a prey species only when the prey exceed a certain minimum number. This minimum may result from a fixed number of safe refuges, physically or behaviourally determined, in the environment; or may occur because the predator finds it unrewarding to search for prey below a fixed density and moves off elsewhere. Errington showed that a given area of range in Iowa could support a relatively fixed number of bob-white quail in winter irrespective of the autumn population, while the surplus birds were taken by predators. A similar story applies to the red grouse in Glen Esk which have been studied in great detail by D. Jenkins, A. Watson and G. R. Miller. It has already been noted that a territorial system relates grouse numbers to the carrying capacity of the habitat, forcing excess birds to move into less favourable marginal areas. Most of this dispersal takes place during two distinct seasons, from November to December and from February to April, the displaced birds suffer much more from predators than those resident in territories. Knowledge of a territory presumably enables the individual to find hiding places when danger threatens. Of 383 birds individually tabbed which had territories in November the remains of 2% were later found killed by predators, whereas of 261 tagged birds known to be displaced from territories in November as many as 14% were found killed. On high ground (700 m. and above) eagles and foxes accounted for about half the grouse preyed upon, while on low ground (500 m. and below) foxes and hen harriers were about equally responsible for the losses. Table 2 also shows how grouse suffered much more from predators in years when the number known to be dispersing was high. The number of raptors actually hunting in the study area also increased in such years, although roughly a similar total was present in the general area each year. Jenkins et al. presume that the grouse were but one of a number of suitable foods and were only taken when they were particularly vulnerable. In this case the number of predators was not determined by the availability of the specific prey studied; their numbers may have been related to the abundance of all prey animals combined, but this is at present unknown. There are times when it is only the birds dispersed to marginal habitats in ways like those outlined above, that cause economic problems – one case is given below (see here).

      At Glen Esk gamekeepers persecuted the predatory birds and mammals at every opportunity. In spite of this, Jenkins et al. showed that slaughter was not controlling these predators because a similar number appeared every year. They point out that the number of predators expected to be killed on the moors of Perthshire and Kincardineshire today are similar to figures quoted in 1906 by Harvie-Brown. Keepers may well reduce breeding numbers slightly, or prevent the breeding of some individuals in early summer, but the relatively large number of young produced on the estates in question and near by, is always sufficient to make good these losses. In other words, gamekeepers merely crop an expendable surplus of predatory birds and mammals, in much the same way as these predators crop their prey – as a man does when he shoots grouse. In the circumstances, it is probably a waste of time for the keepers to set their traps and patrol their estates in search of so-called vermin. As far as grouse are concerned, the most useful employment for the keeper would be to lend a hand with heather burning and help to improve moor management, for this is really what affects grouse numbers, not predator control.

      It would be a sophism to infer from this account that the activities of gamekeepers have never been detrimental to the birds of prey, but it is likely that in many cases the senseless slaughter has at most accelerated processes brought about by more fundamental changes, particularly in land use or loss of habitat. Once a species declines and becomes restricted in range through lack of habitat it is far more vulnerable to persecution by man. The osprey was probably always rarer as a breeding bird than some authors have implied, as it was restricted by a need for large lochs with good fish populations, but man’s greed and his continuous persecution eventually caused its extinction in Britain at the turn of the century. The marsh harrier, too, has long been persecuted. In one Suffolk locality two or three pairs regularly nested up to 1951, in spite of egg-collectors (the mentality of one of the men concerned is shown by the fact that in one day he took nine clutches of shoveller from the level where the harriers bred to demonstrate clutch and egg-size variation) and shooting by the keeper of a nearby estate. But it was drainage of the marsh to improve cattle grazing that spelt the final doom for the harriers at this site in the mid 1950s; not just persecution, senseless though it had been.

      Unlike the marsh harrier, the hen harrier has proved remarkably adaptable in its habitat requirements: indeed in the Americas it is the only harrier and occupies the combined niche of all the Old World species. In the north of Britain it has increased, despite fairly heavy persecution, and is doing particularly well in the new conifer plantations of Scotland, which are rich in ground rodents. On the whole, the hen harrier seems to be making good the ground it lost in the nineteenth century, when an even more intensive slaughter banished it from the Scottish СКАЧАТЬ