THE BIRDS OF WHALE ISLAND THEIR STATUS AND DISTRIBUTION. J.P. Croxall* and P.R. Millener*

Transcription

1 Tane (1971) 17:53-60 «THE BIRDS OF WHALE ISLAND THEIR STATUS AND DISTRIBUTION J.P. Croxall* and P.R. Millener* SUMMARY A list of species recorded on two short visits to Whale Island in July and August 1970 is presented. Of the 42 species, 22 are sea or shorebirds (4 species breed; 2 others probably do so) and 20 landbirds (14 species breed; 2 additional species probably do so). The results of a small survey attempting to determine the abundance of the landbirds in the five major habitats (grassland, swamp, kanuka scrub, cliff forest, and coastal forest) are also given. The predominance of grassland has enabled introduced seed-eating species to flourish. The impoverished nature of the forest ensures that insectivores are the dominant species here. Fantail (forest),yellowhammer (grassland) and Chaffinch (mainly grassland) are, in that order, the commonest landbirds on the island. INTRODUCTION After the preliminary visit to Whale Island by Field Club members, including the first author, (11-15 July 1970), it was decided that in addition to compiling a species list, members of the main party (16-25 August 1970), including the second author, should also attempt to determine the abundance of land birds on the island. Details of the physiography and vegetation cover of the island are presented in other papers in this volume. SURVEY OF ABUNDANCE OF LAND BIRDS Introduction and Methods In a week of observations it was possible to obtain only a very rough idea of abundance of landbirds on the island. To simplify matters, attention was focused on the major habitats alone; minor habitats and transition zones being ignored. The habitats in which abundance data were recorded were grassland, swamp, kanuka (Leptospermum ericoides), cliff forest and coastal forest. In each habitat the method of survey was used was for the observer to walk slowly more or less directly through the habitat, noting all species seen in the habitat. Birds flying over were also recorded but were not included in the totals for each habitat. Every habitat was revisited as often as time permitted. It was found difficult adequately to cover the large grassland area in one day and it was subdivided into four, more or less discrete areas. These were the grassland on Pa Hill (P), around the camp site and up Camp Valley (C), on the * Department of Zoology, University of Auckland.

2 54 promonta/y at the southeast end of the island (E), and the remaining main cone (M). For similar convenience the cliff forest was also subdivided. One part (A) ran north from the campsite below Pa Hill. The other, a more extensive area (B), ran most of the length of the landward side of the island, south-east of the camp site. Although kanuka is widely scattered over the island the only more or less pure stand occurs in Sulphur Valley and it was here that the abundance of species in kanuka was surveyed. The simple nature of the survey technique used will probably affect the results from the various habitats in different ways, but common to all will be the under-recording of most species. Hence nearly all species will be shown as rarer than they actually are. The tendency of individuals to be recorded in more than one habitat will partly offset this however. Results The results of the survey are presented in Table 1. As well as giving the average number and average total of species recorded in each habitat, it was decided also to include the maximum number of individuals recorded in the habitat in any one survey transect (or in the case of subdivided habitats the total of maxima for one survey of each subdivision). In view of the inevitable under-recording of most species the maximum total may be the more realistic figure. The total number of species and individuals recorded from each habitat is given. It is not possible to distinguish between resident and visiting species, so no great significance can be attached to these figures. Discussion It is not proposed to discuss the actual figures from the table as their accuracy is likely to be sufficient only for indicating the comparative abundance of landbird species. The first part of this discussion will deal briefly with the birds typically found in each habitat, also noting the likely results of the deficiencies in survey technique. Following this a number of more general points will be considered. Grassland This is the most extensive habitat on the island and the greatest number of species and individuals were recorded from it. Visibility is good in the habitat and many species can be recorded at considerable distances. The size and nature of the area, means however that shy and secretive species such as Quail and Hedge Sparrow will be greatly under-recorded. Many of the resident species (e.g. Yellowhammer, Skylark, Quail, and Harrier) are typically birds of open country, and on Whale Island were found either exclusively, or most abundantly, in this habitat. One or two other species (e.g. Chaffinch and Blackbird) are more characteristic of open but wooded country. The presence of numerous bushes and shrubs throughout the grassland probably enables them to find breeding sites and thus occur as resident species in the habitat. Yellowhammer and Chaffinch are the commonest species; Harrier and Skylark probably breed only

3 55 in this habitat. Swamp A small open habitat in which recorded numbers should reflect fairly accurately the abundance of the species present at the time of the survey. The resident population is small, consisting chiefly of Yellowhammer. In July, when most of the dominant sedge has ripe seeds, a large number of seed-eating finches (Redpoll Greenfinch and Goldfinch) were present but these visitors had left by August.

4 Kanuka As a distinct habitat this covers a small area, is uniform in aspect, dense in composition and all species are probably under-recorded in it. Nearly three-quarters of the birds recorded were Fantails, the area having little to offer species which are not largely insectivorous. Most of the other species recorded are probably not resident. 56 Cliff Forest This exists as a narrow fairly open belt of trees around the perimeter of much of the island. Reasonably accurate recording is possible in this habitat which is rather lacking in floral diversity and, as expected, insectivorous species appear to make up the bulk of the residents, with Fantail most numerous. When pohutukawa (Metrosideros excelsa) is in flower it should attract other species, especially Tui and Silvereye, for a while, but the latter species can probably only breed in the area because insects form a large proportion of its diet. That fewer species are recorded in this habitat than in the small kanuka area is offset by the fact that more species are resident in the cliff forest and a greater number of individuals are present. Coastal Forest This habitat, of botanically greatest diversity, is rather restricted, being confined to the top of Pa Hill and to the summit of the island. The forest undergrowth is dense in parts, especially on the steep slopes of the valleys where the presence of the stinging nettle (Urtica ferox) also restricts access. Consequently most species are likely to be under-recorded here. In spite of this the survey shows a species diversity only slightly less than that for grassland, and considering the forest area, a high density of individuals. Although the insectivorous Fantail is the commonest species, Tui was only recorded here and Silvereye occurs at its highest density. Both these species, but especially the Tui, require a substantial amount of nectar, fruit and flowers in their diet. The remnant forest on Pa Hill is less modified than that on the summit and this is reflected in the number of birds present in each. Beach Sandy beaches provide a foraging area for a few landbirds, notably Chaffinch and Starling. The commonest species on the island are Fantail, Yellowhammer and Chaffinch. All other species are considerably less abundant. The Starling, although one of the commonest species, was rarely observed other than in flight, and the figures for its abundance in any habitat are hardly realistic. It possibly breeds in all habitats except the swamps and beaches, and probably at greatest density in the forest. It is hardly surprising that there are no species on the island which feed wholly on fruit and leaves. The extensive grasslands ensure an abundant (if seasonal) supply of seeds and the impoverished nature of the forest favours insectivores rather than fruit, flower and nectar eaters. Morepork and Harrier eat

5 57 small birds and rats as well as insects; Starling and Blackbird supplement their insect diet by scavenging on the beach and by eating berries and fruit respectively. Silvereye feeds on nectar, flowers and fruit to a greater extent than any other species, save Tui, and accordingly is confined to cliff and coastal forest. It is doubtful if the Tui, with its substantial dependence on fruit and nectar is resident on the island, but it is equally hard to see why it should visit the island to sample the rather small supply of these foods. As seeds are a major food resource, it is not surprising that several introduced species of seed-eating small passerines should have been successful colonists. With the substantial modification of vegetation on the island, the dominance, both in species and numbers, of introduced birds is not unexpected. If goats are removed from the island it should be interesting to monitor any changes in the avifauna that result from subsequent bush regeneration. It was mentioned earlier that the simple nature of the survey would result in most species appearing rarer than they actually are. In combination with other factors, notably the short duration of the work, this means that the figures shown in Table 1 only give a rough idea of the actual abundance of landbirds. In spite of these drawbacks it was felt that the data provided by a species list alone would be inadequate to form the basis of even the most general assessment of changes in subsequent years. Although we now have a fairly accurate knowledge of which landbird species occur on each of the many offshore islands from the Three Kings to Whale Island the lack of data on their abundance (the work of Turbott, 1940, being a notable exception) makes it impossible to trace any population changes. In these circumstances it seems that any information on species abundance is better than none. In concluding this section we would like to emphasise that it is still necessary to undertake surveys and census work on offshore islands. Changes in the vegetation and in the density of natural and introduced predators are still occurring, and excellent opportunities to study the effects of these changes on the avifauna will be missed as long as there is no reliable data on population levels. SPECIES LIST The list that follows is intended to summarise the general information on species observed during the two visits. Eudyptula minor (Little Blue Penguin) Breeds on island. 15 individuals were banded from a colony located between Camp Bay and Sulphur Valley. Diomedea melanophris (Black-browed Mollymawk) One offshore in July. Diomedea salvini (Grey-backed Mollymawk) One offshore in July. Macronectes giganteus (Giant Petrel) One seen on crossing in August.

6 Daption capensis (Cape Pigeon) One seen on crossing in August. Pachyptila sp. (Prion sp.) Two seen on July crossing. 58 Puffinus carneipes (Flesh-footed Shearwater) Seen on all crossings. P. bulleri (Buller's Shearwater) Seen on crossing in August. P. gavia (Fluttering Shearwater) Seen on all crossings and occasionally in flocks offshore. Pterodroma macroptera (Grey-faced Petrel) Several hundred pairs breed on island. Pelecanoides urinatrix (Northern Diving Petrel) Seen on all crossings. Sula senator (Gannet) Regularly seen offshore; largest number observed was250. Phalacrocorax carbo (Black Shag) Two seen offshore in July. P. varius (Pied Shag) Colony of at least ten pairs on island; maximum number observp 1 on any one occasion was 20 adults and 12 juveniles, in August. P. sulcirostris (Little Black Shag) One offshore in July. P. melanoleucos (Little Shag) May breed. 3-4 individuals seen regularly in August. Circus approximans (Harrier) At least one pair breeds; up to four individuals seen. Synoicus australis (Australian Brown Quail) Almost certainly breeds in the grassland, although its shy habits meant that never more than two individuals were seen at any one time. Haematopus unicolor (Black Oystercatcher) Probably breeds on sandy beaches on the island; up to seven individuals Charadrius obscurus (New Zealand Dotterel) M.J. Imber, Wildlife Service, records (pers, comm.) that a pair bred on t main beach in Larus dominicanus (Black-backed Gull) A small colony (c.10 pairs) breeds at the southern end of the island. L. scopulinus (Red-billed Gull) Apparently does not breed, but up to 60 seen in flocks offshore.

7 59 Hydroprogne caspia (Caspian Tern) A few seen offshore and on crossings. Sterna striata (Whitefronted Tern) Up to 100 seen offshore. Ninox novaeseelandiae (Morepork) A pair breeds in the forest on the summit of the island. Halycon sancta (Kingfisher) At least two pairs appear to breed on the island. Alauda arvensis (Skylark) Well dispersed throughout the grassland, where it breeds. Rarely seen elsewhere. Rhipidura fuliginosa (Fantail) The commonest bird on the island; principally found in the forest habitats. Petroica macrocephala (Tomtit) Rare vagrant. One recorded by M.J. Imber (pers. comm.). Gerygone igata (Grey Warbler) Probably breeds, but not common on the island, three being the largest number seen. Recorded from the coastal forest on both Pa Hill and the summit, and from the kanuka area in Sulphur Valley. Turdus philomelos (Song Thrush) Almost certainly breeds on the island and seen in small numbers in all habitats. T. merula (Blackbird) Breeds on the island and seen, more commonly than Song Thrush, in all habitats. Prunella modularis (Hedge Sparrow) Almost certainly breeds in in the grassland areas, a small number of sightings being recorded on each visit. Probably commoner than records suggest. Prosthemadura novaeseelandiae (Tui) Status uncertain but possibly only a visitor to the island. Not seen in July but up to six present throughout the August visit. M.J. Imber (pers, comm.) regards it as a regular visitor. Zosterops lateralis (Silvereye) Virtually restricted to the forest habitats where it is normally seen in small flocks (5-10 individuals). Chloris chloris (Greenfinch) Two seen feeding on seeds in the swamp in July. Visitor. Carduelis carduelis (Goldfinch) Abundant (c.100) in grassland and swamp in July, but fewer than 20 seen in August.May breed in small numbers on the island but status uncertain. Carduelisflammea (Redpoll) Common (c.50) in swamp and surrounding grassland in July, but only about

8 60 10 individuals remaining in August. Probably a visitor to the island. Fringilla coelebs (Chaffinch) The third commonest resident landbird, seen mainly in the grassland areas. Emberiza citrinella (Yellowhammer) The second commonest resident landbird. Very scarce away from the grassland areas, where it is easily the commonest species. Passer domesticus (House Sparrow) Usually seen in the grassland area near the camp site, but not common on the island and its status uncertain. Sturnus vulgaris (Starling) One or two flocks of c.25 individuals seen in July, but rather larger flocks observed in August when it was estimated that c.100 birds were present on the island. Rather surprisingly it was rarely seen during the day, usually being observed near dusk. Assumed to breed on the island. A total of 42 species are recorded from Whale Island and its surrounding waters. Of the 22 species of sea or shorebirds, 5 are known to breed on the island and a further 2 species probably do so. Of the 20 species of land bird, it is almost certain that 14 species breed and 2 other species may also do so. Three species are visitors, and a fourth species a vagrant. ACKNOWLEDGEMENTS The help of S. Beddington in carrying out the survey is gratefully acknowledged. Thanks are also due to all Field Club members, in particular D.J. Bettesworth, who contributed observations. M.J. Imber gave much useful information and his hospitality on the island was also greatly appreciated. REFERENCE TURBOTT, E.G A bird census on Taranga (the Hen). Emu 40: 158-