![Thumbnail: [Volume 77, 1948-49 page 236]](/journals/rsnz/images/rsnz_77/thumbnails/rsnz_77_05_1013_0236_ac_02.gif)
Island Populations.
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.
![Thumbnail: [Volume 77, 1948-49 page 237]](/journals/rsnz/images/rsnz_77/thumbnails/rsnz_77_05_1014_0237_ac_02.gif)
| 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.