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 mainland as a breeding species. In eastern Europe, the pallid harrier has expanded and increased its range from the Russian Steppes, in close association with the spread of agriculture. Thus human disturbance alone does not necessarily disturb birds of prey. This is shown by the distribution and breeding of the osprey on the eastern end of Long Island Sound in coastal Connecticut and New York with little regard for human activity; it nests on artificial man-made platforms as does the stork in Europe.
The difficulties of measuring the contribution of habitat change and the persecution of gamekeepers, skin and egg-collectors to the decline of the raptors is well illustrated in the case of the buzzard. The changing status of this bird has been very carefully documented by Dr N. W. Moore following a survey sponsored through the British Trust for Ornithology. Until the early nineteenth century the buzzard was to be found over virtually the whole of the British Isles. Then a serious decline occurred in East Anglia, the Midlands and much of Ireland in the mid-nineteenth century, followed by some recovery in the twentieth century. Today densities of 1–2 pairs per square mile can be expected in suitable habitats. The decline cannot be attributed directly to the spread of agriculture during the nineteenth century because the species underwent increases and decreases both during times of agricultural advance and recession. Also during this period the rabbit, one of the main foods of the buzzard, became more common. Similarly, more urbanisation took place between 1915 and 1954 when the buzzard was increasing, than during the years 1800–1915 when it was decreasing. Furthermore, in the 1954 survey, which indicated a British population of 20–30,000 birds, the highest buzzard density was recorded on mixed agricultural moorland, rather than in pure forest or on extensive moorland, where nesting sites seem to be in short supply. In fact, Moore attributes the early decline of the buzzard to the game-preservation which boomed from 1800–1914. Convincing evidence is provided by his maps, which show an inverse correlation between areas of intensive game-preservation, judged by the number of gamekeepers per square mile, and the distribution of buzzards. His view is also supported by the fact that the biggest recovery took place during the two world wars, when there was much less game-preservation, and many keepers were fighting a different adversary. However, the early decline of the buzzard in the nineteenth century is also temporally related to a marked decline of sheep farming, particularly in East Anglia, and, as discussed below in the case of the raven and carrion crow, the associated loss of carrion may have provided the initial cause, being only accelerated by keepers. Nor does persecution account for the disappearance of the buzzard from Ireland.
Myxomatosis was confirmed at Edenbridge in October 1953, and from two original outbreaks it rapidly spread until by early 1955 rabbits throughout the mainland of Britain were infected with a 99% lethal strain of the virus (Armour and Thompson 1955). The 1954 buzzard survey was carried out before there had been widespread reductions in rabbit numbers, but already many poultry farmers and shooting men were afraid that the bird would now turn to other forms of prey, particularly chickens and game-birds. The same concern was accorded the fox, but fortunately an investigation of this animal’s feeding habits had been made before myxomatosis by Southern and Watson (1941) and this was repeated by Lever (1959) on behalf of the Ministry of Agriculture in 1955. The results showed that in the absence of rabbits, foxes concentrated on other small rodents which would normally have been their second most important prey; the incidence of poultry or