is remarkable. The explanation that they drifted in with down winds seems unlikely, and instead I wonder whether the existence of anticyclonic weather facilitated a normal adaptation after breeding in the form of a deliberate dispersal north-west, a process possibly truncated in years of less favourable weather.
Man has done so much in a passive way to alter the avifauna of Europe that it seems reasonable to take active steps to reintroduce lost species. Any reservations that this would be unnatural, should be tempered by the thought that the environment we have created is in any case artificial. Probably more pleasure than harm has been derived from the reintroduction of the capercaillie. It would seem laudable to follow up a recent suggestion and attempt the reintroduction of the bustard to parts of the East Anglian Breckland, and to encourage black terns to stay and breed. It is quite a different matter to introduce alien species to a new country, especially without sound biological knowledge. In Britain, some of these introductions, red-legged partridge, various pheasants, little owl, Canada and Egyptian goose and Mandarin duck have on balance improved our bird-life, but the same could not be said of the introduction of the house-sparrow and starling to Australia and North America.
Perhaps a more interesting question to ask here is why the majority of introduced species are unsuccessful. This is part of the much bigger question of what factors determine faunal diversity and enable some habitats to support more species than others. The concept of a niche, which refers to the animal’s place in the biotic environment and its relations to food and predators, should now be widely appreciated. It is a fundamental tenet of ecology that no two species can occupy the same niche in any one habitat, because both cannot be equally well adapted. R. and J. MacArthur (1961) examined numerous habitats at different latitudes for their plant species composition and foliage profile and ‘species diversity’. They found this last to be a more useful measure than the actual number of species because their calculations allowed at a habitat containing 50 of species A and 50 of B to rank a higher diversity than one with 99 of A and 1 of B. (The latter tends to be the farmland situation, the former that of tropical forest.) It turned out that neither the variety of plant species nor the latitude affected the amount of species diversity which instead depended entirely on the variation in foliage height, probably because birds mostly respond to different configurations of vegetation in different layers. This means that habitats of the same structural profile have the same diversity of bird species. In any area, a bird might either feed on all food of a suitable size within a narrowly defined habitat or, alternatively, be selective of food but collect it throughout a wider range of habitats. In other words, birds could partition their food or their habitat. The former has occurred because feeding specialisation brings the greatest advantages and has been favoured by natural selection. Partitioning the habitat would necessitate birds moving from one suitable micro-habitat (say a species of tree) to another, and it would depend on the pattern of the total habitat how much time would be wasted in the process. But adaptation to a comparatively broad habitat structure, for instance, to arboreal or ground feeding, must in turn impose physical limitations which restrict the diversity of feeding adaptations; in practice, bill size and shape is about all that can be much modified to suit the collection of different foods.
From the viewpoint of zoo-geography, the Palaearctic has existed as an entirety for sufficient time to ensure that most niches are filled by highly efficient species. Furthermore, man and birds have lived side by side since Neolithic times, so that the new habitats created by agriculture and man’s other activities have been occupied by the species best suited to them. The same is not true of Australia and New Zealand, which were cut off from the main centres of evolution at an earlier stage, one result being that primitive marsupial mammals were not replaced by the better adapted placental mammals. Birds are less insular, however, and the native avifauna of Australia seems to be the best fitted to occupy the niches available. Thus of at least 24 bird species deliberately introduced into Australia in the past, only 12 have become established. It is significant that only the blackbird has managed to invade native forest, the remainder existing in areas of recent agricultural development or urbanisation. But introduced birds like the feral pigeon, starling and house sparrow are better equipped to occupy the man-made niches than the native fauna, simply because these are species which have already been selected to occupy a man-made environment. New Zealand has an impoverished avifauna compared with Australia on which it has depended for colonisation, and the process is still incomplete. In consequence, fewer niches are saturated in New Zealand, so more exotic species have been successful. Of 130 species originally introduced, 24 have become established, although apart from the blackbird, chaffinch and redpoll which appear to be filling unexploited niches, most are again restricted to man-made habitats. Hawaii, which is even more isolated, and not saturated by a wide diversity of species, must have even more vacant niches, for, according to Elton (1958), of 94 birds introduced 53 have become established, some deep in the native forest.
According to Middleton, the European goldfinch has been successful in Australia and New Zealand only in the man-made agricultural areas, to which none of the native Australian Ploceid finches were adapted. In contrast, the European goldfinch has not been a successful bird in North America because it has virtually the same ecological requirements as the fitter endemic American goldfinch. Only one small colony of European goldfinches became established near New York, though these have since vanished when their habitat was destroyed for building purposes. Again the European house-sparrow has been highly successful in Australia, over roughly the same range as the goldfinch, whereas the introduced greenfinch is more restricted as it has rather more conservative ecotone requirements. It is interesting that this reflects a trend occurring today in Britain; the greenfinch is declining with the loss of hedgerows and woodland edges, while the goldfinch and linnet are increasing.
To return to New Zealand, it is noticeable that the birds which have become pests in agricultural areas, apart from being introduced species as one would expect from the comments above, present the same kinds of problems as they do in Britain. I am grateful to Dr P. C. Bull for allowing me to give details. As we shall see, skylarks (see here) are locally troublesome in Britain to young seedling crops such as lettuce. Near Hastings, N.Z., they and house sparrows have together been responsible for damaging asparagus and other seedlings. Both blackbird and song thrush and also the starling, resort to orchards in the dry season after breeding and cause considerable damage to all kinds of fruit, ripening pears, cherries and grapes. Redpolls do considerable damage to apricot blossom in their search for insects, and blossom searching is a habit which is increasing in Britain (see here). Locally in Britain, linnets peck out the seeds from strawberries (see here), while in N.Z. goldfinches do the same.
Various attempts were made to introduce the rook into N.Z. from 1762 onwards but only 35, liberated near Christchurch in 1873, seem to have thrived. The species occurs in five localities on the yellow-grey earths in the east of the county, generally where cereal growing occurs. Rooks at first increased very slowly but there was a rapid increase between 1935 and 1950, and a final levelling off with the density of birds in their favoured areas becoming virtually the same as that in Britain (around 16 nests per square mile). At Christ-church, the population increased from 1,000 birds in one rookery in 1925 to 7–10,000 in 1947 (13 rookeries), since when the numbers have remained roughly constant with 19 rookeries in use. Until 1926, the rookeries were in eucalypts, probably the favourite tree, but following a disease epidemic which killed these trees the birds changed to pines. Bull points out that the rate of increase of rooks has been slower than that of other introduced passerines and attributes this to their gregarious nesting habits and their need for group stimulation, and to an early shortage of suitable habitats. A feature of N.Z. rookeries is their very large size compared with British ones (rookeries of over 1,000 nests are quite common), and the traditional return to the same nesting sites may partly explain the slowness of expansion. (The large size of rookeries, and difficulties in getting sufficient food locally may also explain why the birds seem to lay, on average, smaller clutches in N.Z.; 3.4 eggs against 4 + in Britain, (see here), though more data are needed to establish the point.) Frequently, only when man actively disturbed these large rookeries
