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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 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.

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* (#litres_trial_promo), 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 (#ulink_900ded3a-2da8-5cff-9e9e-4812f984d83a)), 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 to see starlings, jackdaws, and less often magpies, associating with livestock and even perching on the backs of the animals. They catch the insects flushed from the ground by the animals’ movements or those attracted to the beasts, such as various flies. In addition, they sometimes search the fur for ticks and other parasites, like the tick-birds of Africa. The habit does not cause trouble in this country but in the U.S.A. magpies (a subspecies of the European form, which has a ring distribution extending all round the world) sometimes become more adventurous. Schorger (1921) and Berry (1922) have described how the birds learned to peck open a small hole in the sheep’s back, which they gradually enlarged until they located the kidneys which provided a favoured delicacy. Unshorn sheep on open range were sufficiently protected by the thick fleece, and it was only after shearing, when the animals were confined to untended paddocks, that the trouble began; possibly the birds were originally attracted by small wounds left by the shearers. Even small sores provide sites for secondary attack by blowflies. This kind of damage is reminiscent of the attacks of the kea parrot of New Zealand.

The progression from a commensal to a parasitic association between bird and mammal host is well seen in the red-billed oxpecker in Kenya. These birds feed on the ticks and other insects gleaned from the larger game animals, and help the host by warning it of impending danger. Occasionally, they also make the most of blood clots and fragments of skin from any abrasion or wound and will purposely open up a sore with hammer-like blows to eat the serum and blood discharged. Van Someren (1951) comments that the wounds inflicted on the livestock are smooth saucer-like depressions, 1–3 inches in diameter, which do not suppurate, perhaps because the birds keep them clean. Oxpeckers feed on open sores by nibbling with a scissor-like motion as if squeezing out the blood and serum. The attacked animals seem untroubled and their wounds rapidly heal if protected from bird attack. The dependence of the oxpecker on ticks is emphasised in districts where insecticide dips are extensively used. In these places the bird has declined drastically rather than become more prone to flesh feeding as some people feared. The European starling has also been recorded as inflicting extensive wounds on cattle in Texas, by pecking at warbles (McCoy 1941). Apparently the birds were first stimulated to attempt this mode of feeding when more normal food supplies were inaccessible through frozen ground.

CHAPTER 4 (#ulink_d3cfb05b-46b0-5a81-a43b-9b629627181c)

BIRDS AND FORESTS (#ulink_d3cfb05b-46b0-5a81-a43b-9b629627181c)

TO see oak woodland today in anything like its natural state, with a field layer of bluebells, bracken and brambles, and a shrub zone of holly and other small deciduous trees, one must seek out places like the New Forest or the Forest of Dean, or search for other isolated remnants, mostly derived from secondary plantings, which are scattered in small patches throughout the land. Oak in something like this condition had covered much of Britain for around 4,000 years, to be almost totally destroyed since the enclosures of the last 300 years. At sites in Breckland, pollen analysis reveals a decrease of tree pollen in about 3,000 BC followed by the appearance of grass ling and heather and there seems no doubt that Neolithic man was able to clear some quite extensive areas, to produce the heathland that exists today; that this has been maintained does of course owe much to sheep and rabbit grazing. These early efforts at forest clearance, though effective, were certainly made on a very local basis. The open nature of oak woodland allows the development of a rich associated flora of shrubs and herbs supporting a consequently diversified fauna; this diversity has also been facilitated by a long period of establishment enabling a wide variety of plants and animals to reach Britain from Europe. In contrast, beech woodland only spread to Britain at the end of the Atlantic climatic period, roughly at the time when the English Channel was formed and the land bridge with Europe was severed. With the beech came the sweet chestnut, hornbeam and various poplars, and a few other species. None of these have become so widely established as the oak, partly because they are more demanding in their physiological and ecological requirements. In times of much more severe preboreal climate birch was the dominant vegetation, and large areas reminiscent of these ancient times still survive in many parts, particularly in the north of Scotland. Native Scots pine, which preceded the southern deciduous forests, is now confined to the ‘black woods’ of the Highlands. This, like birch, provides a more uniform habitat than oak and supports a less rich, but none the less interesting, avifauna.

After systematically spoiling almost all the native woodland, man has replanted the landscape with comparatively few small stands of hardwoods, a fairly considerable acreage of trees in orchard and hedgerow, and an increasing area of exotic conifers, from the Japanese larch to the north American Sitka spruce and Douglas fir. These new woods have been colonised in varying degrees by our native sylvan birds. Deafforestation, reafforestation (often with new and exotic species) and afforestation (the planting of trees where none previously grew, at least in recent times) have certainly altered the bird fauna, as we shall see in the next chapter. Here our task is to discover whether the birds have any clear effect on man’s interests. The more general question of the role played by birds in the ecosystem is much more difficult to answer, because the interactions involved are complex and not readily measured. There have been many general statements made about the whole plant and animal community, which can be neither refuted nor proved. For example, it is often claimed that some birds, by eating forest seeds, may hinder natural regeneration and, conversely, that birds actually help to distribute seeds and fruit. While it is undoubtedly true that many plants have evolved dispersal mechanisms which rely on animals (the mistletoe is a good example), no quantitative data on such inter-relationships are available. Turček considers that jays, which are specialist feeders on acorns, are practically the only agents able to move acorns uphill. Mellanby (1968) has also considered the role of animals in this respect and disagrees with the many ecologists who consider that most animals prevent natural regeneration of oak woodland. Those animals which destroy the most acorns by feeding on them also appear to be of most importance in causing oak regeneration. Turček also has data to show the importance of birds, particularly jays and blackbirds, in disseminating the sweet cherry in some spruce forests in the central Slovakian mountains. During the summer a mean of 18 casted seeds and 7 seedlings were found per square metre, most within 50 metres of the fruiting tree. Similarly, nutcrackers transport the seeds of Pinus cembra