R. K. Murton - Man and Birds

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 up to nineteen ravens were seen associating with the sheep, but though paired they seemed to be non-breeding birds, and were probably immatures not yet holding territories. Ratcliffe considers that established pairs keep very much to their own territories and do not associate in flocks in this way. It is very likely that these bold attacks on lambing sheep are largely made because birds excluded from the large territories, which must normally contain ample stocks of carrion, are short of food.

Good shepherding in the hills of Britain is a tradition that goes back for centuries. It has always included burying carcases which attract predators and cause disease, the regular and frequent surveillance of lambing flocks and help for ewes in difficulties and for weakling lambs – bad cases are even brought down from the hills. Ridpath concluded that any trouble could be greatly reduced by returning to these practices. If and when control is really needed (the raven is rare and is protected by law) it should be aimed only at the birds causing the damage. It should not involve indiscriminate killing of all the corvids in the area, most of which probably cause very little harm.

Unlike the raven, the carrion crow has increased considerably throughout Britain after suffering a marked suppression from the 1860s until the early twentieth century. A decline in the intensity of game-preservation after two world wars has certainly been a big factor, but it is clear that the crow’s feeding habits have enabled it to become re-established in areas now unsuitable for the raven. It seems likely that it has benefited from changes in agriculture and is the best adapted avian scavenger of the new farm environment. That its numbers are still increasing over most of Britain is shown by a B.T.O. inquiry recently conducted by Prestt (1965) for the period 1953–63. It is probably significant that the only region where no increase has occurred over the last ten to fifteen years is East Anglia, where game-preservation remains most intensive.

Burgess (unpubl.) recently organised a survey of carrion crows over an area of 6,000 acres, near the confluence of the North and South Tyne rivers in Northumberland. This is predominantly a pasture area, lying 2–300 feet above sea level, and consists of large fields surrounded by untrimmed hedgerows with many mature trees. The survey involved the destruction of all occupied nests that could be found in mid-May and a repeat of this operation in late June and August, partly to check for repeat nests or those previously overlooked. The first search for nests was begun in April. For the whole area, including those overlooked in the first operation, there were about 103 occupied nests in May 1961, 134 in May 1963, 128 in 1964 and 137 in 1965 (old nests which were never used were noted and totalled about as many nests again in each year). The results indicate a breeding population averaging one pair to about 50 acres, excluding an unknown number of non-breeding individuals. They also suggest a remarkable constancy in the size of the breeding population in different years, a feature also noted for the raven by Ratcliffe. Population fluctuations in birds of prey, including some corvids, seem to depend largely on the number of non-breeding individuals, partly because the size of a breeding territory seems less flexible than in many bird species, and sets a relatively constant limit on the size of the breeding population, which thus remains stable over long periods. This is not to deny that if long enough periods are considered, or different habitats, the size of the territory is ultimately adjusted to the food supply available. As virtually all successful breeding was prevented in 1961 by the nest destruction, it is clear that this had no depressing effect on the subsequent breeding population, a result in keeping with other similar studies and to be expected.

Because of their smaller size, crows seem less of a danger to ewes at parturition and to young lambs, but because of their large numbers and wider range they provide a greater potential threat to the sheep flocks. In Wales, Ridpath saw two carrion crows kill a ewe and her lamb during delivery, and during his three-week watch he also recorded 30 abortive attacks on sleeping lambs, where the crows crept up to the animal and then pounced at the head or tail base.