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.
It is most distressing for a shepherd to contemplate such savage attacks; to see his defenceless lambs with their eyes pecked out obviously rouses deep emotions which make it hard to keep the problem in perspective. It is difficult to obtain objective and unexaggerated estimates of damage. Burgess (1963) did try to overcome this problem and organised an inquiry covering 155 selected hill-farms in Cumberland and 59 in Westmorland in 1962, after a good deal of publicity to ensure that all incidents would be reported. These farms between them supported some 82,000 ewes and in all 16 attacks on ewes were reported (0.02%). In nearly all cases the ewes attacked were in some difficulty, trapped in snow drifts or hedges, lying on their backs or giving birth. About two-thirds of the attacked ewes did not survive, but a little over half of these were already sick, many suffering from staggers. On the same farms there were 69 attacks reported on lambs, approximately 77,000 being at risk. About half the attacks were made on live lambs (0.04% of lambs at risk) while half again were fit lambs that should have survived. Allowing for unreported cases, the loss of lambs due to crow attack must be well under 0.5% of those at risk.
A pilot survey was conducted in Argyllshire between May and July 1964 (Gailey in litt.) by officers of the Department of Agriculture and Fisheries for Scotland. On 50 farms holding 48,390 ewes and 36,292 lambs*, losses attributable to hoodie crows were 192 ewes (0.4%) and 366 lambs (1%). Losses due to all predatory birds (including eagles, ravens and great black-back gull) amounted to 1% of the total sheep stock, hoodies being responsible for 0.65% of this total. Again, this is a very low percentage of damage particularly as this area of Scotland is generally reckoned to suffer the highest level of crow damage. In Argyll, the average mortality of ewes is 7.4% from November to July and 1.6% from July to November, according to McCreath and Murray (1954). These authors give lamb losses as 13% between birth in April and marking in June, and 5% between marking and sales in September. The sheep stock for the county of Argyll is approximately 450,000 breeding ewes and 338,000 lambs. The application of these results to the whole county would mean a loss of 2,926 ewes and 2,195 lambs to hoodies, which at £5 and £3 per head respectively at first suggests £21,000 worth of damage. But this is the kind of calculation made by the farmer and is quite unjustified. It can be calculated from McCreath and Murray’s mortality data that around 16,000 ewes (3.6%) and 60,800 lambs (18%) would die in the county between April and the September sales, a level of normal wastage far above that of the damage attributable to crows.
The level of damage to sheep seems to be markedly similar in widely separated areas. A survey in Radnorshire, Breconshire and Montgomery in 1969 showed that under 0.01% of 114,751 ewes at risk were attacked by crows, while 0.6% of 119,680 lambs at risk were attacked, the figure becoming 1.4% if only farms where attacks actually occurred are included (K. Walton, in litt.). In Australia, Smith reckoned that avian predators were responsible for the death of less than 2% of the lamb crop, and other Australian studies indicate that the live lambs attacked are already ailing; many have no milk in their stomachs and seem not to be receiving proper maternal care. The work of Alexander et al. (1959) in Australia has shown that sheep in their lambing flocks react relatively little towards foxes, more towards crows and most of all towards dogs. Unlike foxes, crows make very determined efforts to attack lambs.
As already discussed (see here), losses are not additive in these circumstances and it is likely that deaths caused by predatory birds simply improve the survival chances for the remaining animals, so that the final yield is unaffected. There would have to be a very much lower death-rate of sheep and lambs from natural causes before it could be accepted without qualification that predatory birds were depressing the output. The survival of sheep must depend largely on the carrying capacity of the hill, and an effective reduction in sheep mortality would best be obtained by improvements in land management. In large areas of Britain overgrazing and bad land management have been responsible for much sheep carrion, and this in turn supports the predator population. It is this sort of problem that needs evaluation and the immediate answer is not an out-and-out war on the birds. In some circumstances these birds may indeed be troublesome, even allowing for natural losses – but biologists cannot accept the extrapolation of damage costs, as in the example above.
In Britain it is common
