Collins New Naturalist Library. R. Murton K.

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      FIG. 13. Seasonal changes in the number of wood-pigeons (top figure) or domestic pigeons (lower figure) in the diet of the goshawk in Germany. The dotted line is based on Murton, Westwood & Isaacson 1964 and represents seasonal changes in the population size of the wood-pigeon. Goshawks take more pigeons when the population size of their prey is swollen by a post-breeding surplus of juveniles, domestic pigeons having their peak breeding season earlier than wood-pigeons. (Based on data in Brüll 1964).

      Table 3 summarises the diet of two other birds of prey, the sparrowhawk and goshawk. Apart from demonstrating how two closely related species differ in their food requirements, enabling them to co-exist in the same deciduous woodland habitat without competition, the table shows the importance of the wood-pigeon in the diet of the goshawk. The fact that the goshawk is slightly larger than the peregrine and is also a woodland species accounts for its ability to take those larger pigeons which the peregrine rarely utilises. Many people have suggested that the goshawk should be encouraged to settle in Britain to help control the wood-pigeon population, but there is no evidence that it would take a sufficient toll to be effective, for the same reasons that eagles and harriers do not control grouse numbers. Fig. 13 supports this view by showing the proportion of wood-pigeons in the prey of goshawks at different seasons, against seasonal changes in wood-pigeon numbers. Clearly wood-pigeons are mostly eaten at the end of the breeding season when many juveniles are available, and in mid-winter when population size is still high. In spring, when the goshawk could potentially depress population size below normal – and hence really control numbers – it turns to other more easily captured prey. In contrast, feral and domestic pigeons breed earlier in the year and have a population peak in June; this is when they are most often caught by goshawks.

      Neolithic husbandmen were doubtless familiar with the presence of ravens and crows near their domestic animals, long before biblical shepherds were tending their flocks aware that these birds were a potential menace to a young or weakly animal – the eye that mocketh at his father … the ravens of the valley shall pick it out (Proverbs 30: 17). Predacious habits and black plumage, burnt by the fires of hell, long ago made the crows prophets of disaster. A suspicion of such augury still persists among those who today think it appropriate to hang corvids and birds of prey on some barbed wire fence or makeshift gibbet; while these crucifixions may well release human frustrations, they do nothing whatever to deter the survivors (see Chapter 12).

      Ravens are no longer widely distributed throughout Britain as they were in medieval and even more recent times, but there are still frequent complaints from hill farmers and shepherds in parts of Wales, northern England and Scotland that ravens, and hooded or carrion crows, sometimes kill or maim lambs and even weakly ewes. According to Bolam (1913) sheep, mostly in the form of carrion, comprise the major part of the diet of ravens in Merionethshire, sheep remains being found at least three times more frequently in castings than remains of any other food item (these including rabbits, rats, voles and mice, moles, birds, seashore and other invertebrates, snails and large beetles and some vegetable remains of cereals and tree fruits). Similarly, E. Blezard (quoted by D. Ratcliffe 1962) found sheep remains in over half the castings he examined from birds in northern England and southern Scotland, the next most important item being rabbit, which occurred in only a quarter of the castings. The examination of castings probably underestimates the importance of rapidly digested invertebrates or vegetable foods, but it is clear that sheep (probably as carrion) are an important food source, although the raven, like the crow, is very much an omnivore and carrion feeder. There is no reason to doubt that the raven had similar food habits in the past, when it occurred throughout lowland Britain; in fact, we know that shepherds in Suffolk around 1850 were bitterly hostile to the bird – ‘five were among Mr Roper’s sheep at Thetford in August 1836’. Like the buzzard, the raven was a reasonably common breeder in Norfolk and Suffolk until about 1830, but it declined markedly thereafter, coincident with the rise of intensive keepering, and it had vanished by the end of the nineteenth century. While continued persecution was doubtless responsible for the final elimination of the bird, and was also probably responsible for making the carrion crow very rare in the second half of the nineteenth century, other factors doubtless contributed to the initial decline. Loss of carrion is usually given as the cause, and it seems likely that it was specifically the loss of sheep carrion that was responsible. In Norfolk and Suffolk this coincided with the period of active enclosure, particularly that of waste land and sheep walks from 1800 to the mid-nineteenth century. According to Arthur Young, half Norfolk yielded nothing but sheep feed until the close of the eighteenth century, when with enormous speed – enclosure was mostly achieved in twenty years – the land was covered with fine barley, rye and wheat. The rapidity with which enclosure was completed is manifested in west Norfolk by straight roads and compact villages, the result of planning on a large scale, whereas in the east of the county the winding lanes, isolated churches, farms and homesteads derive from centuries of slow economic evolution.

      Although improvements in hygiene, a lack of carrion and the extensive use of firearms may have eliminated the raven from most of lowland Britain, in relatively undisturbed areas, like the Welsh and Scottish Highlands, its density has probably been altered less. But even in such areas man has much reduced the upland forest habitat of the species and caused it to depend on cliffs for breeding. For more recent times, Ratcliffe (1962) has been able to show that breeding populations in four areas he studied have not dropped by more than 14% since 1945, and average only 6% below the maxima ever recorded. Some increases may even have occurred in areas where the bird previously suffered intensive persecution; in the Scottish borders tree-nesting, but not rock-nesting, has increased since 1945, indicating an increase in local populations which are again able to exploit traditional nesting sites. In Ratcliffe’s four inland study areas the average size of a raven’s territory ranged from 6.6 to 17.6 square miles (in these same areas the breeding density of the raven was about 2½ times that of the peregrine) but higher densities may occur in favourable coastal areas, for example, four pairs in two miles of cliffs in Anglesey. In Pembrokeshire, R. M. Lockley estimated the raven population at 80 pairs in 1949. In 1953 M. G. Ridpath (Report to Ministry of Agriculture, Fisheries and Food, 1953) searched 25 miles of cliff, in the 140 miles of apparently suitable coast-line, and found an average of one breeding pair every two miles.

      Ridpath spent three weeks (220 hours) between 9 and 30 March, 1953, watching a lambing flock of about 1,500 sheep in the Prescelly Mountains, Pembrokeshire. During this period he saw two lambs killed by ravens, and in addition nine other attacks on lambs and eight on adult ewes. Attacks on lambs were concentrated on the eyes, lips, umbilical cord and anus. In one case two ravens persistently attacked a four-day-old lamb in spite of the mother’s efforts to defend it. At first the ewe managed to ward off the birds, but eventually one of them managed to peck at the lamb, at which point the mother walked away leaving the birds to finish the kill. In many other cases when attacks were first witnessed, the ewes were active in defence of their young and successfully repulsed the birds.

One of the local farmers, an experienced observer, showed Ridpath a young lamb which he had seen killed by a raven as it was being born. By weight and appearance it did not seem to have been a weakling. Both ravens and crows are certainly attracted by the afterbirths at lambing time, and sometimes newly born lambs are not readily distinguished. Lambs are most vulnerable during the moment of delivery (especially if parturition is at all difficult, or twin births occur) when the mother cannot guard them, and for two or three days afterwards. Weaklings are attacked most often, as they are easier to kill than healthy lambs, which make vigorous attempts to escape. Ravens and crows (and for that matter golden eagles in Scotland) rely mostly on carrion, which is fairly common owing to the larger number of sheep which perish in the rigorous hill environment (see below). Attacks on sheep are most frequent at lambing time and during the winter months when parties of ravens or crows are attracted to the supplementary feed put out in the vicinity of the sheep flocks. There is reason to believe that much of the trouble is caused by non-breeding or immature individuals of either species. Thus during Ridpath’s study СКАЧАТЬ