by David Lack (1937)
This short paper is mainly a summary of two earlier ones [10, 11], and is written partly because the conclusions then put forward seem to have been somewhat misunderstood.1
The limits of a bird's distribution are probably often determined by such factors as climate, food, nesting sites and natural enemies, though the summary by Moreau  shows how little work has as yet been done on these lines. But in addition to these admittedly important "direct" factors, there are others of a different nature, here termed psychological factors, which involve the bird's mental reaction to its environment. A bird's behaviour is usually described as primarily instinctive, based on inherited patterns of behaviour which, though capable of some modification, are on the whole rigid. It is this rigidity which makes psychological factors of such importance in distribution. Examples will now be considered.
The Blue Tit (Parus c. obscurus) does not breed in woods in which there are no suitable nesting holes. Hence it is absent from many copses and young plantations where it occurs commonly outside the breeding season and where it will breed readily if nesting boxes are put up. This shows that the nesting holes are the sole missing feature. The factor concerned is not the absence of nesting sites but of a particular kind of nesting site. The Blue Tit could successfully colonize young plantations if its mental equipment (a psychological factor) enabled it to build a nest like a Long-tailed Tit (Aegithalos c. roseus) or a Lesser Redpoll (Carduelis f. cabaret). Of numerous similar examples, one more may be given. The Common Guillemot (Uria aalge) nests on flat ledges and the Puffin (Fratercula arctica) in holes, not the other way round, as a result of which each species is restricted to certain types of coast.
In Iceland the Harlequin Duck (Histrionicus histrionicus) is confined to rocky swift-flowing streams, where it feeds on Ephemerid and Phrygænid larvæ by turning over stones in the streams. The Harlequin is absent from the lakes, in which many other species of duck are common. This is correlated [[p. 131]] not with its specific diet but with its specialized feeding habits. Hugh Wormald informed me that they are the hardest of all ducks to rear in captivity, as the ducklings "swim about poking their heads under every inequality in the ground under water and are extremely difficult to get to eat anything". Once this peculiarity was realized, they were successfully reared on ant pupæ, freshwater shrimps and insects, a by no means specialized diet. But this habit would clearly prevent successful breeding in the lakes; a psychological restriction.
In south-eastern England the Tree-Pipit (Anthus trivialis) often occurs in a habitat identical with that of the Meadow-Pipit (A. pratensis) provided at least one fairly tall tree is present. The tree is used neither for feeding, nor for nesting. The Tree-Pipit occurs away from trees outside the breeding season, and in a tree-less locality I have found one using a telegraph pole. The tree is used solely in song, but nevertheless is an essential element in the bird's habitat. It is interesting that the Meadow-Pipit has a very similar song but normally dispenses with a tree, though occasionally using one when present. Lack and Venables  relate a similar example for the Stonechat (Saxicola torquata). Others could be given, and probably in most species which sing from a high perch, a song post is an essential of the breeding habitat, even when trees or bushes are used for no other purpose.
Habitat selection illustrates a more general, and probably the most important, influence of the psychological factor on bird distribution. In many land plants and marine animals, dispersal is effected by chance scattering. Those seeds or embryos which happen to encounter a favourable habitat flourish, and the rest die. But in birds, the procedure is different. When the summer visitors arrive in England in the spring, they do not settle in all types of habitat and attempt to breed, but each species proceeds to its own specific habitat, in much the same way that each species will later select a nesting site of the specific type. Clearly, in birds, each species (instinctively) selects its habitat, in contradistinction to plants, in which the habitat selects the species.
Habitat selection is of value since it means that the bird will settle in a habitat similar to the ancestral one, usually, therefore, a favourable one. One might expect recognition of the specific habitat to depend on the visually prominent, and [[p. 132]] not necessarily the essential, features of the habitat. An otherwise favourable habitat which lacked these recognition features would not be colonized, and the chief evidence for a psychological factor in habitat selection is the absence of species from habitats apparently well suited in essentials but not in superficial appearance. For instance, what prevents the Reed-Bunting (Emberiza schniclus) from breeding in the typical habitat of the Yellow-Bunting (E. citrinella) and vice versa? What normally restricts the Rock-Pipit (Anthus spinoletta petrosus) to the rocky foreshore, while the Meadow-Pipit breeds on moorland? What restricts the Wood-Warbler (Phylloscopus sibilatrix) to woodland areas almost devoid of undergrowth? In the last case, food, nesting sites and song perch seem clearly ruled out. Howard  describes how Wood-Warblers returned to a locality a second year but soon departed, a procedure which could be correlated with an increase of the undergrowth and nothing else. For further examples, see [10, 11], and more could be given. Brock  seems the first to have realized the importance of "environmental bias" in bird distribution, and soon after the present writer's first publication , Moreau [14, 15] independently put forward similar views on the importance of "subjective factors" (to my mind an unsatisfactory term) from a study of distributions in Tanganyika. Finally, Howard , from a quite different approach, that of territory, concludes that a bird knows its natural home, such knowing being inherited, very few mistakes being made.
Apart from habitat selection, three explanations have been put forward for the often marked differences in distribution between closely related species: food, differential adaptation and inter-specific competition.
It is often assumed that a species is limited to a particular habitat because its food is so restricted, but the reverse explanation may be the true one, i.e., the bird is limited to particular foods because its habitat is restricted. For example, Jourdain  notes that the sole difference between the food of Reed- and Yellow-Bunting is that the former eats mainly marsh plants and insects, the latter those of drier situations. Since both species have such a varied diet, one cannot suppose that this slight difference in food could cause the marked difference in distribution; it is clearly the other way round. Similar considerations apply to many other cases of distribution. The evidence of Collinge , Jourdain , McAtee  and others, shows that most birds have an extremely [[p. 133]] varied diet. But food preferences undoubtedly exist, and at times limit distribution. Thus Formosof  has shown the dependence of the Siberian Nutcracker (Nucrifraga caryocatactes macrorhynchus) on the cedar nut2, and Howell  that of the Everglade Kite (Rostrhamnus sociabilis plumbeus) on a particular snail. But such instances are uncommon, and leave the cases particularly under discussion completely unaccounted for.
Structural differences not infrequently occur between closely related species. For instance, the hind claw of the Tree-Pipit is shorter and more curved than that of the Meadow-Pipit. In this and other cases the structural difference may well be adapted to the habitat, but it does not follow that the adaptation is itself the cause of the difference in distribution. A priori it seems more probable that, in most cases at least, the difference in distribution preceded the adaptive difference. And there are many closely related species in which such adaptive differences have not been described and may well not exist. Adaptation will obviously account for many limits to distribution, for instance, the restriction of aquatic birds to water, but seems quite inadequate to account for many distributions, particularly those of closely related species under discussion.3
Possibly in some cases, closely related species are differentially adapted to their respective habitats to an extent sufficient to mean that, if their distributions were left to competition, each would be more successful in its present habitat, though there is no definite evidence for this, and it seems unlikely to apply to most cases. But there is no evidence that inter-specific competition of the type required by this view occurs. Further, this is quite inadequate to explain why the species do not normally attempt to breed outside their own habitat, an objection which also applies to the other two alternatives.
[[p. 134]] DIFFICULTIES OF HABITAT SELECTION.
It therefore seems probable that each species selects its own habitat, guided by recognition features which are not necessarily in themselves essential to its existence. But habitat selection of this type is an extremely difficult factor to investigate. First, once the principle has been accepted, there may be a tendency to invoke it for any case of distribution for which there is no ready explanation, and its existence is almost impossible to test directly. Secondly, though it may be the factor preventing a species from attempting to breed in a particular habitat and may in some cases (e.g., the Wood-Warbler cited) be the sole factor involved, in other cases there may also be other factors which would effectively prevent successful colonization if the bird did attempt to do so. Habitat selection being admitted, the presence of these other factors might be overlooked. Thirdly, it is not a factor capable of much analysis. Probably the bird recognizes the habitat "as a whole" or by a combination of features, not by any one taken singly. In the Wood-Warbler a habitat exists similar to that occupied by the bird save in one particular, namely, increased height of undergrowth, hence one of the features essential to the bird's recognition can be determined. But such cases are rare.
The problem of why closely related species so often select different habitats is of great interest, but any answer must, in the present state of our knowledge, be extremely speculative. I have attempted  an explanation for certain Passerine species, but this need scarcely be repeated here. In brief, it is suggested that the conditions of habitat selection may, themselves, have led to the segregation of species in some cases.
VARIATIONS FROM THE NORMAL.
Just as individual birds occasionally build a nest that is atypical for the species, so they occasionally break away from the specific nesting site, feeding habit or habitat. This sometimes enables them to breed in areas which would otherwise be uncolonized. For example, Ticehurst  notes that on the shingle area of Dungeness, where its usual nesting sites are absent, the Wheatear (nanthe . nanthe) nests under tins, and even under the roots of a gorse bush or in a depression in the open. Other cases could be mentioned, and as an instance of breeding outside the typical habitat, Harrisson and Lack  found that the Rock-Pipit bred on rocky moorland away from the shore on St. Kilda. That such local or abnormal occurrences are often successful is [[p. 135]] further evidence that the restricting factor was psychological and not a direct environmental factor. One must probably expect such occasional modifications where a behaviour factor is involved, and they add one more difficulty to the investigation of psychological factors.
OTHER ASPECTS OF THE PSYCHOLOGICAL FACTOR.
The writer has been specially interested in habitat distributions. But the psychological element also comes into other fields of bird distribution. As is well known, many species tend to return to breed in the locality where they were reared. Probably this habit alone has been sufficient in many cases to promote segregation, leading to the formation of geographical sub-species. Physical barriers undoubtedly assist such segregation, but in many cases are, in themselves, insufficient, and could readily be surmounted by such mobile animals as birds. Indeed, one sub-species may even pass regularly through the breeding grounds of another on migration, as in the case of the White Wagtail (Motacilla a. alba) and Iceland or Greenland Wheatear (nanthe . leucorrhoa) in Britain. In these last cases the factor primarily limiting distribution is clearly psychological.
In quite another field, the habit of aggressive territorial behaviour at times sets a limit to the density of breeding pairs below that which essential requirements would have permitted, as shown, for example, by Venables and Lack  for the Great Crested Grebe (Podiceps c. cristatus), in which one pair claimed a much larger territory than any others.
Finally, the psychological factor influences those factors which are apparently most definite. The limits of temperature fatal or deleterious to a bird can be measured experimentally, but, as Kendeigh  has pointed out, birds tend to move elsewhere before these temperatures become operative. The same applies to the quantity of food needed by a bird. Discomfort is often the important factor in the field, and this cannot readily be measured.
Various environmental factors directly limit bird distribution, e.g., food, nesting sites, natural enemies, climate. But psychological factors are also important. Birds react to discomfort, which modifies the direct effects of starvation and adverse climatic conditions. Aggressive behaviour may limit the population below that which food and nesting sites could support. The habit of returning to breed where reared causes desertion of other suitable areas frequented on migration. Birds keep to specific nesting sites, feeding habits [[p. 136]] and song perches, and will rarely modify them. Most important, each species instinctively selects its habitat, in which it is probably influenced by the visually prominent, not necessarily the essential, features.
Psychological factors are extremely difficult to investigate experimentally, and to analyze, and their existence cannot be established in the convincing way that the effects of direct environmental factors can be demonstrated. But though extreme caution must be taken in postulating them, their importance in modifying bird distribution is undoubted, and they severely complicate all investigations of the factors limiting bird distribution.
 BROCK, S. E. (1914). The ecological relations of
bird distribution. Brit. Birds, VIII., 30.
1. See for instance Nicholson , Ticehurst
. [[on p. 130]]