Bird Studies at Kaikoura To Help Rare Chatham Island Robin

Forest and Bird, Issue 186, 1 November 1972, Page 10

Bird Studies at Kaikoura To Help Rare Chatham Island Robin

THE New Zealand region has two species of robin which, though having some similarities in appearance and behaviour to the British bird, are not in fact even distantly related to the English robin redbreast.

THE mainland species is divided into three subspecies—North Island, South Island, and Stewart Island—of which the last two are much alike. All have a dark, slaty grey upper plumage with lighter undersides; but the North Island bird is smaller than its South Island relative and has a white breast, light streaks on its upper surface, and a mottled throat, whereas the South Island robin has a lemon-yellow breast and a yellowish white abdomen and lower tail coverts. The Stewart Island subspecies is slightly darker than the South Island subspecies. The other full species—the Chatham Island black robin —is, as its name implies, entirely

By

G. E. Coster,

Wildlife Service, Department of Internal Affairs

brownish black. This robin, which has the dubious distinction of being one of the world’s rarest birds, was first discovered in 1871 on Mangere Island and adjacent Little Mangere Island, in the Chathams group. Now it is confined to the scrub and coastal forest of Little Mangere Island alone, where the

estimated number of birds is between 60 and 70. Population Vulnerable Though this population has apparently been able to hold its own and remain fairly constant at least since 1936, it is vulnerable on two fronts: firstly, because of its low numbers and, secondly, because its survival depends on a very small and isolated habitat from which it has not been possible to banish the spectre of human interference. One fire on the island could wipe out the species overnight. The disturbingly real possibility of the untimely extinction of the Chatham Island robin has caused considerable concern to the Wildlife Service of the Department of Internal Affairs. As yet it has not been possible to buy and reserve Little Mangere Island, but, with Mangere Island having - recently been created a reserve (its purchase was made possible through a large contribution from the Royal Forest and Bird Protection Society), from which all browsing mammals have now been removed (latest reports indicate that revegetation is proceeding very satisfactorily), it is hoped eventually to extend the range of the robins to this and possibly other islands. The main task facing the Wildlife Service is to find ways of increasing the numbers and range of the birds without prejudicing the delicate balance of the present population. Three methods have been considered, and trials on robins elsewhere will have to be made before any attempt is made to manage the fragile remnant on Little Mangere. (Difficulties associated with the isolation, rugged terrain, and delicate forest floor of the island preclude a lengthy study. Simply to walk is both difficult and destructive because of the high density of petrel burrows.) Transfer of Eggs The first method is a transfer of eggs from robin nests on Little Mangere Island to the nests of tomtits (a closely related species) on Mangere Island. It certainly has the advantage of causing minimum disturbance to the Little Mangere population (the birds involved there would lay again), but creates certain other problems. Among these problems are finding the nests of both species at the right time and the fact that tomtits frequently nest in sites where egg

transfers would be difficult. Moreover, if tomtits successfully reared robin chicks, the question would arise of disturbances to robin behaviour producing a race of birds unable to breed normally or able to hybridise. For these reasons this scheme has been given no further consideration. The second method is to release captivebred birds on Mangere Island and other islands. Because of the difficulties surrounding the breeding of small, insect-eating birds in captivity (which would require the same number of birds as a direct transfer) and because it would be necessary to be sure that aviary- stock could adapt to wild conditions, this method might be tried only after a second wild population has been established. Transfer of Birds to Mangere Island The . third method, easier and more likely to succeed than the first and less expensive and risky than the second, is a direct transfer of a few birds to Mangere Island. Before it can be started, however, the numbers and

breeding success of the Little Mangere robins must be determined so that an estimate can be made of how many juveniles or adults could be removed and still permit the population to maintain itself. Habitat Requirements The birds’ habitat requirements and how satisfactorily they are met on Mangere Island and the effect the dense sea bird colonies have on insect production on the two islands must also be known. In addition, the best time for capture and release must be ascertained and consideration given to such associated factors as time of pairing, breeding, and highest mortality and the period of peak condition in the birds and the sexing of juveniles. Finally, it must be determined what, if any, the effects would be on population levels of a mixture of robins and tomtits. The existing tomtit population on Mangere Island may pose a threat to introduced robins; therefore a careful study of populations of these two species as they occur both together and separately is very necessary. All these problems are heightened by the difficulties touched on earlier of conducting an intensive study on Little Mangere Island itself. There are, however, some limited opportunities for direct study. If a sufficiently large proportion of the birds was colour banded, valuable information could be obtained from observations made on a single visit either during or following the breeding season. Nevertheless, before any studies on Mangere or Little Mangere take place, research will be concentrated on mainland and island robin populations to make the most use of the unavoidably short visits which can be made to Little Mangere. Study Site In May of last year Dr J. A. D. Flack, a scientist of the Wildlife Service, who is primarily responsible for the overall planning and development of New Zealand robin studies, began such a research programme in an area of some 300 acres of scrub on the north-east bank of the Kowhai River at Kaikoura. This locality is for several reasons particularly suitable as a study site.

Situated between about 200 and 400 ft above sea level, it is probably at the lowest altitude of any region outside Fiordland containing mainland robin populations; and though some exchange does occur, it is more or less isolated- from other robin communities by encircling farmland. Because of the low growth of the vegetation the task of observing birds and finding nests is made fairly easy, and the simple relief of the country greatly assists general movement and the making of transects. The prime aim of Dr Flack’s study is to assess production, mortality, and movements within the population and relate this information to the problems of conserving the black robin. The main technique used at present is individual colour banding, and about 200 birds have so far been marked in this way. Most have been later observed, often over many months, and the data recorded have enabled an increasingly clear and accurate picture to be built up of the territorial boundaries, breeding, and general activities of a large number of birds. Pertinent Facts Emerge Already, some pertinent facts have • come to light. It has been found, for instance, that the breeding capacity of the robins may be reduced by certain behaviour which occurs between juveniles and between adults and juveniles. This may well be accentuated in smaller, more restricted areas such as on Little Mangere Island, where the removal of juveniles could result in more fruitful breeding. It has also been discovered that the breeding season of the Kaikoura population begins in mid July and ends in February—an unusually long period. Much valuable practical knowledge and experience of safe and efficient methods of trapping and handling has been gained as well. Naturally, there is still much to be learnt and to provide the degree of certainty needed in this project, studies have to be, and will be, extended over a long period. The robin population at Kaikoura has survived both direct and indirect modification of its habitat through land clearance, farming, and flood control and the introduction of exotic mammals. The greatest density of robins occurs in an area easily accessible to the public, which makes the population a public amenity; and its closeness to Wellington,

Christchurch, and the University of Canterbury Edward Percival Marine Laboratory in Kaikoura means that intensive studies can be made of robins and other birds. Indeed, except for those on a few island reserves, no other robin population sharing these features exists in New Zealand. Unique Area For both scientific and aesthetic considerations this unique area must continue to remain unspoilt. Three organisations — Royal Forest and Bird Protection Society, the New Zealand Ornithological Society, and the Wildlife Service — have been involved in varying degrees in its presentation. The area is owned by the Marlborough Catchment Board, which has plans to convert it from native to exotic forest to help meet the costs incurred by the flood relief programme. This, with engineering works for a proposed drainage scheme, could be extremely harmful to the robins, depending on where the works were carried out and the methods used.

To preserve most of the robin population would involve setting aside only a small acreage from the cutting and planting programme (with no additional costs to the scheme), and the Wildlife Service has made this submission to the Marlborough Catchment Board. The service has also suggested a series of precautions to the board which would enable the drainage scheme to proceed with the least damage to the robins’ habitat. First Study of Kind The present study at Kaikoura is the first of its kind for this country. Apart from providing essential information for the future management of the Chatham Island black robin, it is also producing broader insights into more general problems associated with native birds, which in turn could mean more effective management of both mainland and island populations. It is presumptuous to assume that the future of any species will always be bright. The South Island saddleback appeared to be doing very well on the southern Muttonbird Islands,

(where it was confined) south of Stewart Island, until the sudden and unexpected arrival of the black rat so reduced its numbers that it reached the brink of extinction. That the story had a happy ending is now past history.

The Wildlife Service, by transferring birds to other rat-free islands in the area was able to save the saddleback, which is now in a most encouraging position. The North Island saddleback faced a more subtle threat to its survival; not from rat plague, but from its vulnerability in being confined in a range to a single island —Hen— in the north. Realising the consequences

should rats strike there, the Wildlife Service again made transfers of birds to neighbouring islands to safeguard against this possibility. There are close parallels between the dangers which faced the saddleback and those which are facing, or could face, the Chatham Island black robin. The saddleback transfers were a complete success and there appears to be no reason why this method should not produce equally gratifying results if it is eventually applied to black robin. To that end, experimental transfers are being made of small numbers of robins into new habitats on the mainland and on a number of islands.