in surrounding areas, often with a rapid increase in total bird numbers in the district. As in Britain, rooks uproot seedling peas and corn, take ripening peas (and pumpkins) and maize and are also partial to walnuts.
The number of closely related birds which can live in the same habitat without competing for food depends to a large measure on the degree of stability within the environment. Marked fluctuations occur on English farmland, not only because of the changing seasons, but also because ploughing, harvesting and other farm operations impose drastic changes. As a result, the farmland birds occupying the various niches available for ground-feeders show a wide character displacement; we find a plover, three passerines (rook, starling, lark), a partridge and a pigeon, other species being only transient visitors, or primarily dependent on other habitats. No bird can afford to be too conservative in its niche requirements in a fluctuating environment, while the need for each species to show more tolerance reduces the number of ecologically isolated forms. Therefore, we should expect modern farm mechanisation, which enables whole farms to be ploughed within a fortnight, to be detrimental to bird life compared with the old methods which ensured some degree of stability by leaving land fallow and by transforming stubbles into bare ground more gradually. Klopfer and MacArthur (1960) have similarly emphasised that the major factor accounting for a decrease in the number of species away from the tropics, while the number of individuals of each species increases, does not result from a decrease in habitat complexity, but to a decrease in the similarity of coexisting species. The principle can obviously be extended to any situation where man simplifies the environment.
An important feature of complex ecological communities is that interactions between members damp out oscillations in the numbers of any one species (see here) and so help to introduce a high degree of stability and energy utilisation. For one thing, available food is more fully exploited, which is not the case in arctic environments for instance, where considerable seasonal changes occur. Hence, the amount of energy needed to maintain a stable community is less than that required for an unstable one. Man’s activities have tended to reduce complexity and introduce monotony, through monocultures of crops, uniform stands of trees, or rows of similar houses. In consequence, the animals inhabiting these environments usually fluctuate much more than those of more complex ecosystems, often to the extent of becoming pests (see here). One feature of stabilisation is that natural selection can favour anticipatory functions – for example, the breeding season of northern birds has become approximately geared to seasonal daylight changes – in unstable environments opportunism must set more of a premium. Because more energy goes into maintaining fluctuations in simple ecosystems, often short-term fluctuations, these systems offer more scope for rational exploitation, giving more production per unit of biomass. In general, pestiferous birds and game species can be cropped very intensively, but the corollary also applies in that the energy needed to counteract fluctuations, the efforts of pest control, must often be so considerable as to be impracticable. On the other hand, mature and complex ecosystems can be disturbed relatively easily. In the Eltonian food chain the predators at each level become rarer and larger, because the energy passing from link to link is only in the region of 10%-20%. This not only sets limits on the number of links in a food chain (five seems to be the maximum) and rules out the possibility of a super-predator but makes the top predators particularly vulnerable to small but cumulative changes in the food chain. The loss and increased rarity of so many of the birds of prey depends not so much on persecution, but on the reduced complexity of the environment through human ‘progress’. Clearly our future policies should not concentrate too much on bird protection per se, but rather on the creation and maintenance of as much diversified habitat as possible.
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.
