The long-term or ultimate value to a species in settling in an appropriate habitat will depend on the bird’s ability to find suitable food and produce surviving progeny, and this ability will be conditioned by the structural and behavioural adaptations of the species. The immediate or proximate factors which determine how a bird chooses an appropriate habitat are unlikely to involve these same factors. Instead, natural selection has enabled each species to respond to immediate signals, which can be reliably taken as indicators that other more basic needs will be satisfied. In this way a bird which lives in oak woodland might respond to the configuration of an oak tree, because natural selection will favour this appropriate response provided it leads the bird to find in the oak woods all the various foods which are appropriate to its needs and feeding adaptations. Natural selection can favour the emergence of appropriate proximate responses which are anticipatory.
In practice, it is generally agreed that birds respond to a range of releasing stimuli which combine to provide the best cues. A meadow pipit may respond innately to open country, thereafter to specific elements of the habitat, such as the height of grass, the presence of song posts and nest sites. These considerations are important when man radically alters the habitat, without necessarily altering its food value. As various features of the environment combine to produce a response, they need not always be present in the same proportions, and some may even be absent, for a response still to occur. Species vary in the capacity to respond when some stimuli are absent. Within any area intraspecific competition ensures that the most favourable sites are filled at the start, after which less complete habitats can be occupied. At low population densities, when, for example, a species is at the edge of its range, only the best habitats are occupied (stenotopy) whereas when population explosions occur, marginal areas are also utilised (eurytopy). This is well illustrated by the wide range of nesting sites accepted by the various gulls which have undergone a spectacular population explosion in Britain – nesting colonies occur on rocky and sandy sea shores, estuarine and freshwater marshes, inland lakes, and on moors and fells.
It becomes clear how an originally montane bird like the house martin should come to accept the sides of houses for its nest site instead of cliffs. Similarly the absence of a species from what appears to be a suitable habitat may be attributable to the absence of some apparently trivial factor which must be satisfied. Already in south Europe it is known that the presence of electric and telephone pylons and cables in otherwise open country facilitates colonisation by species like the collared dove and various shrikes.
The psychological response of the reed bunting to a limited range of habitat cues seems to have been the only reason for its past restriction to wetland habitats and its ecological isolation from the yellowhammer. But, as will emerge later, this segregation is no longer maintained and the invasion of yellowhammer habitats by the reed bunting is possibly the result of a genotypic change which has removed this psychological restriction. Another explanation is also tenable. Although most habitat recognition is innate, birds are able to reinforce or even modify, to a variable extent, these innate responses by learning processes. By this means, adults often return to traditional areas, even though these change drastically, whereas young birds breeding for the first time avoid entering habitats which do not release appropriate responses – either innate or acquired by imprinting in early life. For instance, Peitzmeier (1952) found that in a study area in Germany the curlew typically nested only in boggy areas and avoided surrounding cultivated land. When the marshes were drained and cultivated, the adults not only remained faithful to the area, but after learning its new characteristics, also spread to other tilled land which before they had avoided. It could be that this situation has applied in the case of the reed bunting. Peitzmeier attributes the further spread of curlews in arable environments to the imprinting of young which have been reared in these new habitats. Such processes explain why local populations of birds come to acquire unusual habitat associations – for example, the stone curlews which used to nest on the shingle of Dungeness and Norfolk beaches, or yellow wagtails which breed in fields of growing potatoes. Peitzmeier accounts for a remarkable and fairly sudden change in the nesting habitat of the mistle thrush in the same way – originally confined to continuous woodland dominated by conifers it began, in 1925, to nest in parkland-type habitat, and in small groups of deciduous trees in cultivated country. That very recent changes may have occurred in the habitat of an animal must always be remembered when trying to understand their adaptations – they may have evolved in conditions quite unlike those in which the birds are seen today.
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