Man and Birds. R. K. Murton

      FIG. 12. Number of nestling kestrels (top) barn owls (middle) or sparrowhawks (bottom) ringed each year as a percentage of all nestlings ringed under the British Trust for Ornithology scheme. The ease with which observers find nestlings to ring is assumed to be an index of the populatuon at risk. The kestrel and barn owl feed on the same small rodent species and it is noticeable that their numbers fluctuate in parallel in a manner reminiscent of the classical predator–prey curve depicted in Fig.11a. In contrast, the sparrowhawk feeds on small birds (see Table 3) and its numbers do not fluctuate in the same way. The suggestion of a decline in sparrowhawk numbers is almost certainly a true indication of the changed status of the species due to contamination of its food supply with persistent organochlorine insecticides. The risks of such contamination are very much less for species feeding on small rodents.

Simple systems of this kind are theoretically liable to change to the kind shown at C in Fig. 11, where the oscillations between predator and prey become self-destructive and lead to the extinction of one or the other. Feeding patterns like those shown above density 3 in Fig. 9, where increased prey density is not compensated by an increased predation (in practice more animals would usually move in to take advantage of such good feeding conditions), tend to produce violent fluctuations. One reason that such oscillations rarely occur depends on the complexity of natural ecosystems as was discussed in the previous chapter (see here). Prey is effectively isolated in groups so that if one group is accidentally exterminated it is re-populated in a density-dependent way according to its own food supply; predators tend to be less efficient at very high prey densities; and some prey have refuges enabling them to escape predation. These factors plus the existence of more than one kind of predator all help to dampen the kind of expanding oscillation seen in Fig. 11c.

      When the percentage of predation at first increases with rising prey numbers, there is a high probability that oscillations will be damped as in Fig. 11b. If the numbers of an insect increase, a proportional increase in the amount of predation by birds could bring the system back to its old level. There is good evidence from L. Tinbergen’s researches that this is what certain insectivorous birds may achieve in preying on forest insects in the manner shown in the sigmoid part of Fig. 9; fluctuations in prey density can be reduced and predator–prey oscillations damped, so reducing the risk of an infestation developing. But it is clear that if insect density rises beyond the level where the predation curve is S-shaped in Fig. 9, that is, if the prey achieves densities where a smaller proportion is taken with rising density, then the predator could not be held to have a regulating effect. As will be discussed in Chapter 4, birds cannot control an insect plague once it has developed, but they may help prevent it developing in the first instance. From the point of view of pest control or conservation one general lesson following from the above is that predator–prey interactions will be most stable in environments with a diverse structure supporting a wide variety of predators and prey, as for example, natural undisturbed oak woodland. Monocultures of introduced conifers would be expected to provide unstable conditions. Voute’s (1946) observation that outbreaks of insect pests are commoner in pure than in mixed stands of trees is, therefore, of considerable interest and adds weight to the suggestion that forestry policy should aim at intermixing deciduous trees in conifer woods (see here).

      It can often happen that a predator takes only a fixed number of the prey with which it is in contact, satisfying its food requirements and allowing the surplus prey to escape. Again, this is an unstable situation which cannot last, either because predator numbers would eventually increase leading to new relationships, or because if the same predators persist in their attacks they will eventually exterminate the prey. Examples are the temporary concentration of birds seeking the invertebrates disturbed by a farmer ploughing a field, the gathering of swallows and martins to feed on the insects blown from a wood in strong wind, and the birds which gather round a locust swarm. Here, the number of prey eaten depends on the number of predators that chances to arrive on the scene, and is not a function of prey density. The scale of losses inflicted by birds on locust swarms seems usually slight. Around a small locust swarm in Eritrea, which covered about ten acres, Smith and Popov noted two or three hundred white storks, many great and lesser spotted eagles, and several hundred Steppe eagles, as well as smaller numbers of black kites, lanner falcons, marabou storks and other species. Shot and dissected storks each proved to have eaten up to 1,000 locusts. But most observers agree that this scale of predation has a negligible effect. More important is the suggestion (e.g. Vesey-Fitzgerald 1955) that birds may be useful in preventing a rapid build-up of locust numbers. Recent evidence from the Rukwa Valley indicates that this is not the case. First, because the preferred feeding habitat of the birds mostly concerned – white stork, cattle egret and little egret – is the short grass area of the lakeshore, whereas the locusts prefer and breed in the long grass associations covering much of the plains. Second, locusts are most abundant from March–June, when bird numbers are low, and decrease for other reasons in July when large numbers of immigrant birds arrive.

      When man preys upon wood-pigeons by shooting them on their return to their roosting woods, the number he kills increases slightly when the total population increases, but the percentage shot declines. Each man shoots at the passing flock but can potentially kill only two birds because he uses a double barrel 12-bore gun. More flocks pass when pigeon numbers are high, enabling a higher total of birds to be shot, but the flocks are also much larger and a smaller proportion of each can be killed. Hunting methods impose a limit on a man’s kill, and the only way to achieve a higher rate of predation would be for more men to shoot or for each man to use a faster firing, or otherwise more efficient gun. The first possibility is limited by social considerations; the number of men interested in shooting, either for sport or for monetary gain, is restricted because today there are so many more outlets for pastoral relaxation and the financial reward for shooting pigeons is low. The battue shoots did not usually begin until the end of the pheasant shooting season, in late January and early February, because only then were gameowners prepared to let pigeon shooters wander over the estates. They ended in early March when shooting at dusk on the longer days interfered with the other attractions of evening, the village dance or local hostel. The alternative of using a more lethal weapon would be opposed to the arbitrary code of sportsmanship current in Britain today, a code which imposes operose conditions on the ways animals can be ‘taken’ – a euphemism for ‘killed’. In days when cumbrous muzzle-loading guns prevailed, shooting birds sitting on the ground was acceptable, but fashion changed to ‘shooting flying’ during the eighteenth century as guns improved, and today shooting a sitting bird is unthinkable. Today a similar pretentious scorn is poured upon the American repeater, just as it was upon the double barrelled 12-bore when it first appeared. As Markland (1727) says in Pteryptegia or The Art of Shooting Flying:

      he who dares by different means destroy

      Than nature meant, offends ’gainst Nature’s law.

      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.