Problems Relating to the Birds of New Zealand's Offshore Islands
This short paper is an elaboration of part of my account of the birds of Little Barrier Island, which is to appear in the July, 1947, issue of New Zealand Bird Notes. There I have referred to certain ecological conditions affecting the birds of island forests, and emphasised that these birds are not typical of forests on the mainland before European settlement. The problem is essentially ecological,
for it refers to differences in populations rather than in species or subspecies. This paper, however, in order to provide a balanced discussion of insular tendencies, refers also to more general problems of speciation.
Island Species and Subspecies.
Non-adaptive differentiation: Insular differentiation, resulting in distinct forms (generally subspecies) has long been familiar to the taxonomist. Until recently no satisfactory explanation had been found for the obviously non-adaptive nature of many island characters. Sewall Wright (1940) and others have now shown that any new character which may arise has a relatively greater chance of survival in the small isolated population of an island. An example of such characters is provided by the grey wing patch and rump of the Chatham Island pigeon (Hemiphaga novaeseelandiae chathamensis), which although apparently of no adaptive significance, form the taxonomic basis upon which this subspecies is separated.
As Mayr (1942) has pointed out, another genetic basis for non-adaptive differences in isolated populations is provided where the entire population has apparently been derived from a single pair.
As a result chiefly of the “Sewall Wright effect,” non-adaptive distinctions tend to accumulate on a small island. The degree of differentiation has been shown to depend upon the size of the island; and must ultimately be governed within broad. limits by environmental and population factors. Again, isolation, especially on the nearer offshore islands, must be interrupted in many species by interchange with the mainland.
Adaptive differentiation: The development of characters directly adapted to local conditions is probably at the same time as effective as on the mainland; and as Huxley (1942) points out, may even be carried to a further pitch in isolated populations. As interesting paper by Marshall and Harrisson (1941), although directed primarily towards proving the non-adaptive nature of many taxonomic characters, provides a record of apparently adaptive tendencies common to many species on Espiritu Santo, New Hebrides, but absent on the Australian continent. These tendencies are towards “sedentary habit, lack of sociability, decrease in voice, stronger nests, smaller clutches, and food specialization.”
Turning to the characteristics of island populations as distinct from species, I shall refer in detail to birds of the offshore islands: Little Barrier Island, Hen Island, and the Poor Knights group.
Influence of area upon presence or absence of species: Area alone would appear to play an important part in determining the species represented on islands of similar vegetation. There is for every species a minimum effective population. The accumulation of deleterious characters, or of those too uniform to provide for. adaptation to environmental change, may result in the elimination of species present in small populations (Huxley, 1942; Mayr, 1942). It is thus possible that the pigeon (Hemiphaga novaeseelandiae), with a diet including large quantities of seasonal fruits, could not on the 480 acres of the Poor Knights establish the minimum number of breeding pairs. The effect is accentuated by the nature of the vegetation, which is a coastal scrub, with some coastal forest, not rich in berry-bearing trees. In my census on Hen Island (1940) I recorded a density of 30 breeding pairs of pigeons in 75 acres, in an area of coastal forest including taraire (Beilschmiedia, tarairi) and other large-fruited trees.
To take another example, the absence of the tui (Prosthemadera novaeseelandiae) from the Poor Knights list contrasts very decidedly with the abundance of the bell bird (Anthornis melanura), a species with apparently similar food habits. Area may here again be significant, in this case probably associated with behaviour in relation to territory. The combined effect of area and vegetation probably answers Buddle's difficulty (1941) in accounting for the absence of tui and other species on the Poor Knights.
It would follow from these considerations that the forest populations of the larger islands should include the greater number of species; and should as the result of both area and more varied environmental conditions reflect more closely those of mainland forests. This is supported by the following comparison of the representation of thirteen species on three islands or groups.
|Little Barrier Island||Hen Island||Poor Knights|
|(Area 7,000 acres)||(1,175 acres)||(480 acres)|
|1. Red-fronted parakeet (Cyanoramphus novaezelandiae)||1. Red-fronted parakeet||1. Red-fronted parakeet|
|2. Bell bird (Anthorms melanura)||2. Bell bird||2. Bell bird|
|3. Kaka (Nestor meridionalis)||3. Kaka||3. ——|
|4. Grey warbler (Pseudogerygone igata)||4. Grey warbler||4. ——|
|5. Pied tit (Petroica macrocephala toitoi)||5. Pied tit||5. ——|
|6. Fantail (Rhipidura fuliginosa)||6. Fantail||6. ——|
|7. Tui (Prosthemadera novaeseelandiae)||7. Tui||7. ——|
|8. Saddleback (Creadion carunculatus) [formerly]||8. Saddleback||8. ——|
|9. Rifleman (Acanthisitta chloris)||9. ——||9. ——|
|10. Robin (Miro australis)||10. ——||10. ——|
|11. Whitenhead (Mohoua ochrocephala albicilla)||11. ——||11. ——|
|12. Kiwi (Apteryx australis) [introduced]||12. ——||12. ——|
|13. ——||13. ——||13. Porzana plumbea|
In view of the successful introduction of the kiwi to Little Barrier, it would be particularly interesting to test its power of survival on Hen Island.
Comparison of total population densities: Direct comparison of density, irrespective of the species involved, gives a general indication of the effect of local conditions on bird populations. Unfortunately, comparison between insular and unmodified mainland forests is no longer possible, but fruitful comparative work is possible based upon a series of offshore islands. By comparison of unit areas it would be possible to gauge the extent to which the few species represented on a small island can concentrate in place of absent species.
My census on Hen Island (1940) represents the only basis for such work in New Zealand so far. The total population density in this case, based upon a count of breeding pairs on an area of 75 acres, was 728 birds per 100 acres.
An unpublished census which I carried out with the help of Mr. P. C. Bull on the Poor Knights in 1940 indicated a lower total density than on the Hen. This may be correlated with the more varied food supply and range of nesting sites on the Hen; and with limitations of increase in the numbers of various species, even in the absence of possible competitors.
To give comparable results, such censuses should be carried out at all times of the year, and if possible on several islands at the same time.
Buddle, G. A., 1941. Birds of the Poor Knights. Emu, 41, 56.
—— 1946. A Second Visit to the Poor Knights. Emu, 45, 315.
Cockayne, L., 1906. Notes on a Brief Botanical Visit to the Poor Knights Islands. Trans. N.Z. Inst., 38, 351.
Cranwell, L. M., and Moore, L. B., 1935. Botanical Notes on the Hen and Chickens Islands. Rec. Auck. Inst. Mus., 1, 301.
Cranwell, L. M., 1937. New Plant Records from the Poor Knights. Rec. Auck. Inst. Mus., 2, 101.
Hamilton, W. M., 1935 (pt. 1), 1936 (pts. 2 and 3). The Little Barrier Island, Hauturu. N.Z. Journ. Sci. Tech., 17, 465 (1), 717 (2); and 18, 557 (3).
Huxley, Julian 1942. Evolution. London.
Marshall, A., J., and Harrisson, T. H., 1941. The Comparative Economy of Closely Related Birds on an Island and a Continent. Emu, 40, 310.
Mayr, Ernst, 1942. Systematics and the Origin of Species, New York.
Oliver, W. R. B., 1922. Little Barrier Island Bird Sanctuary. N.Z. Journ. Sci. Tech., 4, 284.
—— 1925, Vegetation of the Poor Knights Islands. N.Z. Journ. Sci. Tech., 7, 376.
Turbott, E. G., 1940. A Bird Census on Taranga (the Hen). Emu, 40, 158.
—— 1947. Birds of Little Barrier Island. New Zealand Bird Notes (in the press).
Wright, Sewall, 1940, in The New Systematics, Oxford.