CHAPTER 3
THE last chapter was primarily concerned with the factors governing bird numbers and distribution, showing some of the ways in which man does or does not influence the natural balance. The importance of the food supply was stressed. In many cases the food supply can be considered as an independent variable. For example, the quantity of beech-mast varies in different years in response to climatic and other factors and is not initially determined by the birds which use it as food. Nevertheless, the availability of beech-mast profoundly affects the numbers of those species which have to depend on it for food. More bramblings winter in Britain in good beech-mast years. On the other hand, when short-eared owls feed on voles they may themselves become an important factor determining vole numbers. Watson (1893) quotes an interesting example chronicled for 1580. In that year a vole plague developed in the marshes near Southminster, Essex, which so depleted the grasses that cattle died and men were powerless to take any preventive action. The situation was supposedly saved by the arrival of ‘such a number of owls as all the shire was not able to yield; whereby the marsh-holders were shortly delivered from the vexations of the said mice’. When a reciprocal interaction exists between an animal and its food supply, it is termed a predator–prey relationship. The principles involved in such predator–prey interactions are fundamental to almost all aspects of economic ornithology, from the daily routine of the gamekeeper to the effects some forest birds may have on various insect pests. In discussing certain economically important examples it seems desirable to outline some of these principles.
A complex range of variables determines the nature of predation. For one thing the availability of other foods in the environment influences the extent to which a predator concentrates on any particular prey. This also depends on how specialised the predator has become in feeding on restricted categories of prey; kestrels are better equipped to catch ground-living rodents in open country than are sparrowhawks, and these in turn are more efficient at catching small birds in flight in wooded areas. Prey-species have evolved an enormous range of anti-predator devices, the more so if they are subject to intense attack. These protective devices vary from breeding in colonies to the various forms of camouflage and cryptic behaviour and the possession of special defence organs. Some invertebrates and the eggs of some birds are distasteful to predators, and the list could be longer. Whatever anti-predator adaptation has evolved, it is likely that this is also subject to limitations. For example, camouflage can only be effective if sufficient concealing backgrounds are available and when these are saturated surplus animals may derive no benefit from being cryptically coloured. Accepting the existence of all these modifying influences, there are two basic aspects to the response shown by a predator to changes of its prey (Solomon 1949). First, there is the response of the individual predator to changing numbers, or, better, to the changing density of its prey (food), this being the functional response of the predator. Second, predators may respond to increases of prey density by increasing their own numbers through immigration or by breeding, and vice versa, and the change in population size of the predator is the numerical response.
The simplest functional response shown by a predator to changes in food density is depicted in Fig. 8 based on the number of cereal grains eaten per unit time by wood-pigeons according to grain density on stubbles or sowings. The response is simple because the birds have little or no other food choice when they search the stubbles; unlike the related stock dove they do not normally respond to the presence of weed seeds. It can be seen that once grain density reaches a particular threshold the birds’ intake rate cannot be increased; this limitation is imposed because a constant amount of time is needed to pick up, manipulate and swallow each grain. The stock dove’s ability to find weed seeds on stubbles and sowings probably depends on it having shorter legs so that it is nearer the ground. In such ways birds have evolved different feeding mechanisms which are efficient in a limited range of feeding situations.
In most circumstances predators have a choice of prey and the particular item they select depends not only on prey density but also on learned individual preferences. This learning ability introduces a sigmoid stage to the functional response curve as shown in Fig. 9. This type of response curve is much more common among vertebrates and has, for instance, been found to apply to the predation by titmice on forest insects (Tinbergen 1960, Mook 1963), and has been produced experimentally with mammals in the laboratory by Holling (1965). The same curve is also found when bait, in the form of beans or peas, is spread on a grain sowing where wood-pigeons are feeding. This characteristic curve has been explained, as follows, by Leopold in 1933 and more recently by L. Tinbergen. At very low densities of the specific prey (density 1 for curve A in Fig. 9), few or none are found, and the food of the predator consists entirely of other items (100% other prey). As the specific prey density increases, a point is reached when some individuals are found by chance (density 2 in Fig. 9) and for a while the curve rises with density as chance encounters increase. But at some stage, which varies with the attractiveness of the prey, the predator learns that this particular food is available and makes a special effort to find it. The food is now found more often than by chance alone, and this causes the sigmoid stage to appear in the response curve (between densities 2 and 3 in Fig. 9). In the terminology of Tinbergen the bird now adopts a specific searching image for the prey in question. At even higher prey densities the predator again introduces variety into its diet and from now on the prey is taken at a constant rate (from density 3 onwards for curve A in Fig. 9). The level at which the intake rate of prey or the number of prey caught becomes constant depends on its palatability, that is, to what extent it is the preferred food of the predator. A high level (curve B in Fig. 9) would be found for a highly preferred prey (or in the absence of a very good alternative), as when wood-pigeons feed on tic beans spread on a clover pasture. The beans (curve B) are much preferred to clover. If the tic beans are scattered on a grain sowing, the response to beans more closely approximates to curve A because cereals are a preferred food. Nevertheless, the shape and characteristics of the response curve remain unchanged. Buzzards, as will be discussed, feed to a large extent on rabbits, if these are available. In Fig. 9 the rabbit could be represented by curve A and at pre-myxomatosis densities by the vertical line 3, that is, up to 80% of the buzzard’s diet is comprised of rabbits, other prey making up the remainder. Following myxomatosis, which virtually eliminated rabbits for a few years, the buzzards’ diet had to change in favour of other prey, and their feeding response with regard to rabbits could now be represented by the vertical line 1. As with the simple response already considered, it is important to note that above a certain point increase in prey density still does not result in a higher proportion being eaten. There is no reason why the activities of a pest-control operator or a gamekeeper would not obey curves of this kind. When an operator can kill only relatively small proportions of a pest animal, it is necessary to ensure that he does not switch from one pest to another depending on the ease of catching. For example, a rabbit catcher might undesirably be ignoring rabbits at low densities, to concentrate on catching and killing moles, pigeons and other species.
FIG. 8. Number of cereal grains eaten per minute by wood-pigeons depending on grain density. Note that the scale on the abscissa and the actual graph are broken to save space. (From Murton 1968).
Any numerical responses shown by bird predators to changes in the density of their prey can most rapidly be achieved by emigration or immigration; more permanent changes dependent on reproduction must necessarily be slow and delayed, because birds have restricted breeding seasons. Figure 10, based on Mook (1963), shows how the numbers of bay-breasted warblers which settled to breed in certain Canadian conifer forests, after they had returned from migration, varied according to the density of the third instar larvae of the spruce budworm Choristoneura fumiferana. The response did not depend on the reproductive rate of the warblers: it resembles the behaviour of certain arctic birds of prey, like the snowy owl, which settle to breed in the Canadian arctic and in Scandinavia in those years when lemmings are abundant, and is similar
