The Gannet on Cape Kidnappers.
[Read before Wellington Branch, May 7, 1946; received by Editor, May 21, 1946.]
The gannets proper of the temperate and sub-tropical seas (genus Sula) are represented by three species—the North Atlantic-Gannet (Sula bassana), the South African Gannet (S. capensis), and the Pacific or Australasian Gannet (S. serrator). Some authors have treated the two latter species as conspecific with S. bassana (Witherby, 1943).* Apart from differences in plumage, the best-marked distinction between the three species is to be found in their geographical distribution. They are natives of widely separated portions of the globe, but inhabit corresponding zones of latitude.
Although the present paper deals mainly with one New Zealand gannetry, it is appropriate that some mention be made of the distribution of the gannets of Australasia and especially of their location in New Zealand. According to the latest information (K. A. Hindwood in litt., 1946), the Australian form ranges from Tasmania in the south to about as far north as south-eastern Queensland. In the south and west the birds range along the southern Australian coast to about Fremantle in Western Australia. Gannets have also been seen on Lord Howe Island. The main breeding places in Australia are Cat Island, Bass Strait, and the Lawrence Rocks, off Portland, Victoria; they have also been recorded breeding on the Island of Pedro Blanca off the south-eastern coast of Tasmania.
Oliver (1930) gave the first description of distribution and habits of the Australasian Gannet. No attempt has been made so far to assess the present population or even to give a more detailed description of the nesting colonies as has been the case of the North Atlantic Gannet (Gurney 1913, Edwards, Lockley and Salmon 1936, Fisher and Vevers 1943 and 1944). Nor has any explanation been offered as to why the range of the Australasian Gannet extends generally over lower latitudes than its Atlantic counterpart, whose breeding territory reaches much nearer the pole.
[Footnote] * The systematic position of Sula serrator with regard to nomenclature does not yet seem to be satisfactorily settled. Mathews and Iredale (1921) established the new genus Sulita with S. serrator split into two sub-species; S.s. serrator from the Tasmanian Sea and S.s. rex subsp. nov. from New Zealand. Oliver (1930) described the New Zealand gannet as Moris serrator, while Marples (1946) describes this species as Morus serrator. As the present paper deals mainly with ecological and biological problems, we have adopted the specific name of Sula-serrator (Witherby, 1943). Owing to the fact that different terms are being used, we simultaneously suggest a change of the name from “Australian Gannet” to “Australasian Gannet.” Although most individuals of this species inhabit Australian territorial waters, a substantial proportion occurs endemic in New Zealand territory; hence the new name which depiets better Sula serrator's geographic distribution.
The distribution of the breeding colonies hitherto known in New Zealand, with an estimate of their present population, is given in Text-figure 1 and Table 1. According to these data, the present population of ganncts in New Zealand can be estimated at somewhere between eleven and twelve thousand breeding pairs.
|Location.||Population Estimate (pairs).||Year of Estimate.||Remarks.|
|Three Kings||2,000||1934||R. A. Falla|
|Poor Knights||200||1932||" "|
|Great Barrier Island||600||1928||" "|
|Coromandel Islets (Hauraki Gulf)||1,000||1928||" "|
|1,300||1942||R. B. Sibson|
|Gannet Rock (Hauraki Gulf)||1,000||1928||R. A. Falla|
|Oaia Island (off Muriwai Beach, West Coast)||100||1946||" "|
|Alderman Island (Bay of Plenty)||10||1926||" "|
|White Island (Bay of Plenty)||1,200||1926||" "|
|Gannet Island (North-west of Kawhia Harbour)||1,000||Guess—no record||" "|
|Cape Kidnappers||2,000||1922–23||H. S. Cottrell|
|2,837||1945||K. A. Wodzicki & C. P. McMeekan|
|Small Islets, Nugget Point About||30||1936||L. Richdale|
|Solander Island||500||Guess—no record||R. A. Falla|
Cape Kidnappers Gannetry.
This nesting colony is situated at the southern extremity of Hawke Bay (Text-fig. 2) and is claimed to be the only “mainland colony.” The cape was so named by Captain Cook on his voyage in 1769, owing to an attempt by Maoris to kidnap a Tahitian boy from his ship.
According to Cottrell (1923), “the date when these birds first settled at Cape Kidnappers is not known. From an early map of the North Island drafted by Captain Cook, it was seen that although gannetries were marked on Great Barrier and White Islands, no mention was made of their existence on the Cape.”
According to Andersen (1908) the gannet or takapu was well known to the Maoris as a diver, and takapu feathers were used for covering lashings of a canoe. Buller (1888) says that “the Maoris manifest great admiration because of its spirit and dash in catching, and this bird naturally takes a prominent place in the ancient history recording a trial of strength between the birds of the Sea and those inhabiting the Land.” The only available record of interest shown by Maoris in the Cape Kidnappers gannets is the representation made to the Minister of Health in September, 1925, by Mahi Te Atahikoia of Pakipaki. He contended that the Maoris from time immemorial have used the young of these birds as food, and asked the Minister that the right of so using them should be reserved for the Natives. The fact, however, that this claim was made some ten years after the establishment of the sanctuary and that there appears to be no local tradition to support the contention seems to suggest that the gannets at Cape Kidnappers were not taken by the Maoris.
Buller (1897) was the first naturalist to spend a few hours at Cape Kidnappers on December 30, 1896, and the first to give a description of the gannetry and its surroundings; he notes also some of the habits of the birds. At this time the centre of the area (Colony A of this paper) was occupied by nests while the higher ground was used for resting, its whole surface being worn bare by the constant traffic over it. He states also that “at the time of our visit there were probably over a thousand birds nesting there.” Another very brief visit was paid by Cottrell (1923) in January. On this visit also, no count was taken, but “a rough estimate of the number of nests, let us say 2,000, i.e. 4,000 birds.” Cottrell also mentions “a single nesting place.” Since this time there have been no published records of visits by any naturalists to the gannetry, and, apart from very unreliable estimates scattered in the local papers, there is little to indicate the strength of its population within the last twenty years or more.
The idea of establishing the Cape Kidnappers gannetry as a sanctuary originated at a meeting of the Standing Committee of the New Zealand Institute held on January 31, 1914, when the following resolution in regard to the preservation of gannets, on the motion of Mr. Hill, seconded by Dr. Cockayne, was adopted: “That the Government be asked to proclaim a suitable area of land at Cape Kidnappers as a scenic reserve for the protection of a rookery of gannets and to suggest that the Hawke's Bay Philosophical Society be asked to exercise supervision of the reserve.”
Arrangements dragged for some time owing to discussion between the Department of Internal Affairs, the Department of Lands and Survey, and the owner of the land, Mr. F. L. Gordon. After Mr. Gordon kindly presented the necessary land an Order in Council, published in the New Zealand Gazette of September 16, 1915, proclaimed a suitable area of land at Cape Kidnappers as a scenic reserve
for the protection of the gannetry. At the same time a special board was constituted under, Section 2 of “The Public Reserves and Domains Amendment Act, 1914.” At present the reserve is subject to the provisions of Part I of “The Public Reserves, Domains and National Park Act, 1928,” and controlled under a board appointed under Section 17 of this Act. The personnel of this board is as follows: The Commissioner of Crown Lands for the Hawke's Bay Land District (ex officio, Chairman), the President of the Hawke's Bay Branch of the Royal Society of New Zealand (ex officio), and seven other members, all prominent residents of the district. This board has been responsible for the gannetry since and also for the erection of a rest-hut, the building of a track and a fence between the main nesting place and the mainland.
The writers (K. A. W. and C. P. McM.), assisted by Miss D. E. K. Walker (D. E. K. W.) and Mr. J. J. Hancock (J. J. H.), camped at the gannetry between December 16 and 19, 1945, when the count of the breeding birds, most of the observations, photographs, and a short film of the gannetry were made. The film proved most useful documentary evidence of some of the habits of the birds. At the request of the writers, the area was visited again by Rev. F. H. Robertson, of Havelock North (F. H. R.) on February 4 and March 11, 1946, when additional observations regarding the later stages of the 1945–46 season were made.
General Description of the Gannetry.
Cape Kidnappers (Text-fig. 2) is about fourteen miles south from Napier and nearly five miles from Clifton, where the road ends and where the office of the Clifton Domain Board is situated and permits for the visiting of the gannetry are obtained. From Clifton a narrow, rocky beach practicable only at low tide gives access to the gannetry. It runs below cliffs from 200 to 500 ft. high, intersected by deep gullies, some of which carry remnants of the native vegetation. The bulk of the inland part of the peninsula is a hilly plateau deeply intersected by gullies. The pasture cover is Danthonia dominant (Madden, 1940) on which sheep and cattle are grazed.
About four miles from Clifton, the Black Reef (Text-fig. 2), a number of large rocky boulders, belonging to the same geological formation as the cliffs, extends in broken formation at least half of a mile into the sea. The presence of these steep rocks makes this point particularly difficult to cross at high tide and has probably been the main factor protecting the gannetry from vandalism before the establishment of the sanctuary. The Black Reef Rocks harbour one of the three gannetries (Colony C).
About a mile past the Black Reef, at the base of the north side of the cape, there is a rest-hut from which a three-quarter-mile-long track leads to the cape proper and to the two other rookeries (Colonies A and B).
Notes on Bird Life of the Area.
Although gannets form a population which has little if anything in common with other birds inhabiting the adjacent area, it seemed not without interest to record some of the field notes taken during our visits (Table 2).
|Species.||Number and Where Observed.||Remarks.|
|1. Little Blue Penguin (Eudyptula minor)||Single bird on beach between Clifton and Black Reef.||F. H. R.|
|2. Black Shag (Phalacrocorax carbo)||About half a dozen birds including a juvenile on Black Reef Rocks.||Scarce in the district.|
|3. White-fronted Tern (Sterna striata)||3–4 birds seen feeding off shore every day.|
|4. Black-backed Gull (Larvus dominicanus)||Common on beach and also inland.||A flourishing nesting colony of at least 50–60 pairs on steep slopes north of Black Reef.|
|5. Red-billed Gull (Larus scopulinus)||A few birds.||F. H. R.|
|6. North Is. Oyster-catcher (Haematopus reischeki)||Heard single bird flying up coast at dawn.|
|7. Tui (Prosthemadera novaeseelandiae)||Odd birds in gullies.|
|8. Harrier (Circus approximans)||Common, but no more than 2–3 birds seen daily in Kidnappers area.|
|9. Hedgesparrow (Prunella modularis)||A few heard in shrub-covered areas.||Introduced.|
|10. Redpoll (Acanthis cabaret)||A few throughout.||"|
|11. Chaffinch (Fringilla coelebs)||Numerous around plantations and shrub-covered areas.||"|
|12. Goldfinch (Carduelis carduelis)||Plentiful throughout in pairs and flocks.||There is a large amount of thistles on pastures. Introduced.|
|13. Sparrow (Passer domesticus)||A score or so near hut, also feeding on beach.||Introduced.|
|14. Yellowhammer (Emberiza citrinella)||Not uncommon.||"|
|15. Grey Warbler (Gerygone igata)||A few birds in shrub-covered gullies.|
|16. Lark (Alauda arvensis)||Plentiful in Cape Kidnappers area.||Introduced.|
|17. Blackbird (Merula merula)||Common throughout shrubs and plantations.||"|
|18. Thrush (Turdus ericetorum)||Less numerous than blackbird.||"|
|19. Starling (Sturnus vulgaris)||Plentiful throughout both on cliffs, where it probably nests, and in countryside, where flocks of 10 to 40 birds were observed.||"|
|20. Californian Quail (Callipepla californica)||One pair seen.||"|
|21. Mynah (Acrodoceres tristis)||Pair observed near rest-hut; plentiful throughout district near homesteads.||"|
|22. White-backed Magpie (Gymnorhina hypoleuca)||Does not appear nearer to the cape than Clifton.||"|
Cottrell (1923) observed the following species: black-backed and red-billed gulls, white-fronted terns, and silver-eyes.
Of the 22 species of birds observed, five (excluding the gannet) belong to the sea or shore birds. Two species (Sula serrator and Larus dominicanus) were nesting, while a third, Sterna striata, which, according to Cottrell (1923), was then nesting on the Black Reef
rocks has apparently shifted its nesting quarters elsewhere. The landbirds present a rather typical, bird population of pastoral districts of the Hawke's Bay district with three species of native birds (Prosthemadera novaeseelandiae, Circus approxomans and Gerygone igata) still surviving in varying numbers. Finally, it seems evident that some of the introduced birds (Sturnus vulgaris, Alauda arvensis, Acrodoceres tristis) have adapted themselves very well and are probably on the increase.
Description of the Three Nesting Colonies.
The gannets on Cape Kidnappers breed on three different places:
Colony A—the oldest and main colony on the cape itself.
Colony B—on a cliff of the mainland near the main colony.
Colony C—on the Black Reef rocks.
(i) Colony A. The large amount of guano visible many miles from the sea or from the coast indicates that this is the oldest colony (Plate 37, Fig. 1). All available historical data refer to this rookery, which is often claimed as being the only one situated on the mainland of New Zealand.
Two steep conical rocks mark the headland of the cape, which is formed of a line of four steep peaks of Tertiary formation, the height increasing toward the mainland. The two peaks on the seaward end are free from ledges and do not offer convenient nesting places. Nesting is confined to the area between the third and fourth peaks. Through a fence erected by the Domain Board a track on the northern side of the nearest peak leads to a basin-like rocky plateau which is the site of the A Colony. During our visit the fence was in a dilapidated state, and sheep were going to and fro from the mainland into the gannetry, disturbing the birds, especially in the early and late stages of the breeding season. The nesting area consists of a plateau of about two acres about 150 ft. above the sea, with steep cliffs to the sea on the northern and southern sides, and with east and west ends ascending sharply to peaks. The northern and southern edges of the plateau thus form most convenient places for the landing or taking off of the gannets (Plate 37, Fig. 2). The seaward slope to the eastern peak is roughly triangular, with several narrow terraces carrying nests toward the lower part. The slope to the western or inland peak is also triangular and carries a dense nest population right to the top. The main body of nests, however, are situated on the central, roughly square, part of the plateau. The winding, apparently nearly parallel, rows of nests fill the whole of this part. This area is much darker in colour owing to the fact that here the guano is not being washed away so rapidly. Actually, however, the guano layer in spite of the age of this colony is only a few inches thick, probably due to the erosive effect of rainfall and wind.
(ii) Colony B. The relatively recent establishment of this small nesting colony is indicated both by the lack of reference by previous visitors and by the smaller amount of nesting debris and guano deposited. It is situated on a plateau on the mainland about 300 ft. above the cliffs to the north-west of, and less than a quarter of a mile from, A Colony. Apparently the edge of this plateau has been used for a long time as a roosting place for “unemployed birds.” The grass has been destroyed by the droppings of the birds on the level
of the cliff for a length of some 50 yards and the top soil has been eroded for a few yards back from the edge.
This colony is situated in the northern end of the roosting place and has a rectangular shape with nests arranged in crudely parallel rows. Some of these are in the form of low mounds similar to those of the A Colony, but the majority, especially on the fringe of the colony, are made up of a little seaweed on bare, relatively level ground. As there is no fence, it is likely that this colony may occasionally suffer both from trespassing sheep and cattle, or even from human predation.
(iii) Colony C. This colony is about a mile west from the main colony and extends from the ledges of the mainland cliff to five of the larger rocks of the Black Reef (Text-fig. 3). The sixth large rock has no nests and is merely used as a roosting place. Local observers state that this colony is of recent origin.* Since A Colony has obviously reached saturation point, both B and C colonies may be regarded as overflow areas. The presence of a few trees still growing on some of the rocks seems to confirm the recent origin of C Colony.
[Footnote] * A newspaper clipping (Hawke's Bay Daily Mail, July 22, 1939) states that these rocks, were first used as an additional nesting site in 1938–39 brooding season.
The method of “direct counts” in the estimation of the numbers of breeding pairs was used in respect to B and C colonies and also in respect to the less densely populated but well-defined areas on the steep east and west slopes of A Colony. For the densely populated middle section of A Colony (Plate 37, Fig. 2; Text-fig. 4) the following “estimation method” was employed. On this area the nests are arranged in ‘fairly’ well defined and regularly winding rows. The number of nests in ten evenly distributed rows running north and south were counted and the mean taken. This procedure was repeated for the same number of rows running east and west, and an estimate obtained from the products of the means of the two separate sets of counts. Only occupied nests were counted. The aid of field-glasses was enlisted where necessary.
The number of breeding gannets at Cape Kidnappers on December 18, 1945, is given in Table 3. As our visit was relatively early in the breeding season, the majority of young were still in the whitish downy stage or just coming out of it. Hence they could be easily distinguished from adult birds when the counts were made.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Colony.||Group.||No. of Occupied Nests.||Total of Colony.||Per Cent. of Grand Total.|
|C.||1||(Ledges of mainland)||13|
|Text-Fig.||2||(Black Reef rock No. 2)||8|
|(3)||3||(" " " " 3)||30|
|4||(" " " " 4)||300|
|6||(" " " " 6)||18|
|7||(" " " " 7)||7|
On this basis the 2,837 occupied nests had a total population of breeding adults of 5,674 birds. With only a few relatively rough estimates available from previous observers (Buller, 1897, and Cottrell, 1923), it is difficult to express definite views about the trends of the gannet population at Cape Kidnappers. Both Buller's and Cottrell's figures of 1,000 and 2,000 pairs respectively were rough estimates made during a few hours' stay at the gannetry. Nevertheless, even taking their probable inaccuracy into account, there seems to be some increase in the population in recent years. This view is supported by the overcrowding of the main nesting place and the establishment of the B and parts, at least, of C Colony.*
The unreliability of most of the estimates of local unskilled observers as published in the newspapers from time to time may be
[Footnote] * Mr. R. H. D. Stidolph, Vice-president of the Ornithological Society of New Zealand, states (in litt.) that when he visited Cape Kidnappers on January 17, 1931, Colony A was the only one occupied by nesting gannets, while on Colony C White-fronted Terns (Sterna striata) were nesting.
indicated by an estimate published in the Wellington Dominion on October 30, 1945, which states that “the bird colony normally numbers between 8,000 and 10,000 birds.”
Observations on Breeding.
Little is known about the breeding cycle of Sula serrator in New Zealand. Oliver (1930) states that “the breeding season apparently extends over a considerable period, the first birds arriving in August and the last leaving in April.” According to Mr. Allan C. Duff, Honorary Ranger of the Sanctuary, some birds arrive as early as the
(4) Diagram of Three Main Parts of Colony A: 1, steep west (inland) slope; 2, densely populated central flat area; 3, steep east (seaward) slope.
end of June, and the main body is usually there early in July. Other local observers state that the main body of birds reaches the gannetry during August. A reliable observer, Mr. J. Peach, of Gisborne, visited the gannetry on October 28, 1943, and found nesting operations in full swing.†
[Footnote] † The problem of migration of the Australasian Gannet has not yet been investigated. The banding of the North Atlantic Gannet proved that only immature specimens migrate as far south as the western coast of Africa. The bulk of the birds remain in the temperate Atlantic waters.
In order to provide some data for a future more detailed study of this great seabird whose beauty, enhanced with the romance of its surroundings, has accounted for so many students of its Atlantic cousin, and also to obtain some insight into certain points of its biology, three checks of nests were made: the first on December 18, 1945, by the writers and their companions, and a second and third on February 4 and March 11, 1946, by Rev. F. H. Robertson assisted by a friend. The check of nests was made by observers slowly walking across the bottom part of A Colony (Text-fig. 4 and Plate 37, Fig. 2). Each observer walked between three rows of nests and noted the contents of all nests within the three rows. During the December check all birds were fierce and difficult to shift from their nests; the February check, owing to a much greater sensitiveness of the birds to approach, had to be made from the slope. Only nests occupied by an adult (Plate 38, Fig. 1) or a chick (Plate 38, Figs. 2 and 3) were taken into account. The results of these counts are given in Table 4.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Stage of Development of Chick.|
|Per. Cent. of a Total of 435 Nests.||Per Cent. of a Total of 82 Chicks.|
|36||159||1||1||—||—||—||—||—||K.A.W. J.J.H. D.E.K.W.|
|Per Cent. of a Total of 196 Nests.|
Note: The 435 checked nests of Colony A were 19.3 per cent. of the total of occupied nests of this colony, while the 196 nests formed the total of Colony B.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Colony A.||No. of Nests.||Per Cent.||Observer.||Remarks.|
|Empty nests, but occupied by adult bird||33||39.76||"||One nest was occupied by pair of birds.|
|Nests occupied by chick with parent bird||5||28.91||"|
|Nests occupied by chick with out parent bird||19||"|
|All nests except one containing freshly hatched naked chick were empty, but about 150 birds were still located round the colony.|
|A general check of chicks present in the whole of Colony A was also made on February 4, 1946.|
|Stage of Chick.||Number.||Per Cent.||Observer||Remarks.|
|Dead||12||2.59||"||In various stages, but mostly feathered.|
A third check of chicks still present in Colony A was made on March 11, 1946. Only eight chicks remained; of these two still showed traces of down on the head and neck. These were probably the all-white and the partly-feathered chicks on February 4 (Table 6). It is interesting to note that the total population of adult birds remaining was about 300. They occupied roughly the southern half of the well defined dark ground in the trough with a few birds on the eastern hilly slope. The birds were leaving and alighting at fairly short intervals, but Mr. Robertson did not see any feeding operations.
We are well aware that our observations are too scanty to draw any far-reaching or definite conclusions with regard to the breeding of the gannet at this nesting place. Nevertheless, Tables 4, 5, and 6 contain several new facts of great interest.
First, it would appear that the peak of the breeding season this year fell early in December or towards the middle of that month. In the third week of December a little more than half of the breeding pairs still had eggs, nearly 20 per cent. had chicks, while only 28 per cent. of the breeding birds were sitting on empty nests. In view of what is said later, it is not impossible that some of the birds in this latter category may have laid infertile eggs prior to the date of our visit. It appears also that in the beginning of February, 1946, many birds had left; on March 11 the 300 adult birds still present at the Colony A represented only 13 per cent. of the December total of birds.
Secondly, attention must be directed to the curious distribution of eggs and chicks at different stages of development, summarised in Table 4. Together with the equally striking distribution in Colony B, it is obvious that there was not a normal distribution of the new generation from the egg to the partly feathered chick. The situation revealed by the figures was even more strikingly apparent to the observers. Except for an odd newly-hatched chick the new generation consisted of eggs and well-developed chicks with practically no gradual transition in development. The existence of these broad groups—pairs without eggs, eggs, and well-developed chicks—was responsible for the theory, at first proposed, that the Kidnappers gannets were exhibiting a distinct three-phase breeding cycle with the peak of activity occurring in monthly intervals. If true, such a theory would be of great fundamental interest.
The absence of anything but eggs and empty nests in Colony B fitted the theory if it were accepted that this colony was an overflow colony from Colony A and therefore younger. This rather attractive “three-phase-breeding-cycle theory” had to be abandoned in view of the observations of Rev. F. H. Robertson on February 4, 1946 (Tables 4, 5, and 6). During the intervening seven weeks the live-chick population increased from 420 to about 450. Very few dead birds were observed, but at least a thousand eggs had disappeared. In
addition, the 600 odd nests empty in December were still devoid of eggs. Mr. Robertson stated that during his stay on the rock, when a careful watch was kept for several hours, no chick was observed to leave or to return to the gannetry. Analogous observations on the North Atlantic Gannet by Gurney (1913) and others suggest that it is not probable that even the oldest chicks on Kidnappers were sufficiently developed to leave the rock at this date. Equally striking was the virtual disappearance of the 159 eggs in Colony B which were there on December 18.
While it is not impossible that a mob of sheep or some human vandalism could effect these changes in the small and unprotected Colony B, such is out of question with regard to Colony A. The latter is not so readily accessible to stock and the fierceness of the large number of birds concerned would make the destruction of so many nests quite impossible unless organised on a very large scale.
The only logical explanation that appears to fit all the facts noted above is that a very large proportion of the eggs laid by the gannets on Kidnappers during the 1945–46 season were infertile. The gannets' method of incubation between the webs of the feet, together with the frequent turning of the egg with beak and feet on a relatively rough rocky surface (Plate 39, Fig. 1), results in the shell wall of the egg becoming very thin. Incubation of sterile eggs beyond the normal period in this way could cause the eggs to disintegrate and the remnants soon to become unrecognisable. On our request, Mr. Robertson made, during his visit on March 11, 1946, a careful search for fragments of egg shells in and around Colonies A and B. The results are in the following extract from his field notebook:—Colony A: “I was not able to detect shells on the actual gannetry floor,” but “I found several pieces on the grass over the south side”; Colony B: “I found some shell, including one slightly less than half whole shell. This had skin adhering to it still white and well preserved. On the grass on the plateau side of the colony there was a good deal of shell in small pieces, and some collections seemed to indicate that the shell had been crushed about 10–15 yards from the nearest nest.”
On this basis, the three phases to which we have referred merge into one. It may be that subsequent investigations would lead to the abandonment of this second theory which, if true, is of great interest in relation to the maintenance of the gannet species. Such sterility has been reported in other species, notably in the White Stork (Ciconia ciconia) which sometimes as early as in its second year pairs off and occupies a nest. Such nests are either empty or contain infertile eggs (Wodzicki, 1936). Whatever the case, insofar as the gannet is concerned it would appear that the gannets at Kidnappers are unlikely to rear more than 16 per cent. of their number in 1945–46 breeding season to the stage that the chicks are able to leave the rock. We have here a challenge to New Zealand ornithologists.
Colour, Number and Size of Eggs.
The colour of the eggs is, as stated by North (1911), a uniform bluish-white. This colour, however, is usually obscured by a thick coating of lime, and the eggs are generally much soiled by the feet of the sitting bird. The shell of eggs which have been incubated for
some time shows signs of deterioration both owing to the way they are incubated and to the frequent re-arrangement of the egg with the help of the beak.
The Australasian Gannet, like the North Atlantic Gannet, lays as a rule one egg only. There is little doubt that there must exist a correlation between the small size of the clutch and the peculiar way of incubation. The egg is incubated between the webs of both feet, which hold the egg firmly from both sides (Plate 39, Fig. 1).
Several birds were carefully examined, and no brooding patch was found, while in each case the webs of incubating birds were noticeably warmer than one would expect. We could confirm Buller's (1897) observation that “in protecting its naked nestling the old bird would cover it with her broad webbed feet and press it down into the nest as if determined to stamp its young life out …”
Exceptionally, however, two eggs are laid. This has been observed both on chicks and eggs in the middle part of Colony A where, owing to the structure of the nests or age of chicks, an accidental addition of a second chick or egg was out of question. Two eggs, however, are very rare, and on egg basis there were about 0.75 per cent. of such eggs. One set of two chicks in the all-white stage was also observed.
With regard to the size of the eggs there are but few data available. According to Gurney (1913) the gannet's egg is nearly elliptical in shape and “an average one measures 3 inches by 2 inches, which is by no means large for the size of the bird.” North (1911) gives the following measurements (converted to millimetres) of seven eggs from the Australian breeding territory: 78 × 48.3, 75.7 × 37.3, 74.9 × 44.4, 80.0 × 48.3, 78.0 × 46 5, 81.5 × 47.0, 78.7 × 47.2, which gives a mean of 78.1 × 45.6 mm.
One hundred eggs in the Colony A and fifty in Colony B have been measured (Appendix I) and their variation in length and width has been analysed.
The largest and smallest eggs gave the following measurements: longest egg, 88 × 45 mm.; broadest egg, 78 × 50 mm.; shortest egg, 67 × 46 mm.; and narrowest egg, 80 × 43 mm.
Table 7 gives the results of the measurements obtained:
|Colony.||No. of Eggs.||Mean.||Deviation.||Mean.||Deviation.|
|A and B||150||77.57||±||3.10||46.97||±||1.44|
No significant differences exist between the two sets of eggs in either length or breadth.
Size and Structure of Nests.
As seen from photographs (Plate 37), the nests of Sula serrator are built in a way similar to those of Sula bassana, i.e., in winding rows filling all the available space to capacity before another site is occupied by the birds.
The nests are built mainly of seaweed, to which in due course a large amount of guano is added and a mound shape attained. Their height varies considerably, but seems to be lower than that of Australian nests of this species (Mathews and Iredale, 1921). Many of
the old nests on Colony A had a height of up to 6½ cm.; while the majority of nests in Colony B, a recently occupied nesting site, was considerably lower and ranged from a few centimetres down to nests on barren level ground with a few pieces of seaweed scattered in circular formation (Plate 39, Figs. 3 and 4). This seaweed, as stated before, with the excretions of the birds added year by year, eventually forms mounds. This happens, however, mostly on the even ground of the middle part of Colony A, where continuous erosion is probably partly prevented by the contour of the area.
The nest cavity does not vary considerably and is about 23 cm. (9 in.) in diameter. The distance between nests ranges from 70 to 87 cm., being of some 79 cm. in average; this seems to be beyond the pecking distance. Hence about 0 62 square metres (8 square feet) form a single nest's territory.
Display of Nesting Birds.
The easy access to the gannetry gives an exceptional opportunity to obtain an almost complete picture of the breeding behaviour of the gannets. Unfortunately, the short time we had to our disposal did not permit more than placing on record of some observations.
The gannet belongs to species which seem not to possess any sexual dimorphism, and in which both sexes share more or less equally in the reproductive duties of building the nest, sitting on the eggs and feeding the young (Huxley, 1930). As one would anticipate, the display is not confined to the male bird, but both sexes take approximately equal parts. In gannets the display is mutual. Gannets exhibit several different displays, some of which “can be looked on rather in the light of some social customs than as a real part of the breeding” (Stonor, 1940).*
Some of these antics start long before the egg is laid and some continue late into the season long after the chick is hatched. It is also interesting to note that the mating does not occur so frequently and is not immediately repeated as it is in gulls and terns. During our stay at Kidnappers, in spite of a careful watch by all the observers during many hours a day, it was only twice that we were able to witness a mating (Plate 40, Fig. 1), though courtship displays went on almost continuously from dawn to dusk. The following displays associated with breeding were observed:—
Bringing of Nesting Material.
The “Solo Dance.”
The “Greeting Ceremony.”
The Feeding of Chick and Chick Displays.
(i) Bringing of Nesting Material.
This ceremony shared by so many species of birds starts when the “presumed” hen-bird is already sitting on an empty nest. A piece
[Footnote] * Fraser Darling has put forward the concept of “mass stimulation” (Stuart Smith, 1945). He has suggested “that for birds which nest together in large groups a very important factor in the safety of the colony is the need for most of the eggs to be laid at approximately the same time … In order that this may occur, the majority of the pairs must come to full breeding condition at the same time, and this is accomplished by the sexual excitation produced by the mass of birds in the colonial unit.”
of seaweed is, without exception, collected on the surface of the sea and brought by the “presumed” male bird and presented to the sitting partner. These fresh additions to the nest material are often followed by a complete mutual “greeting ceremony.” This display was frequently observed during our December visit to the cape. It is of interest to note that the Rev. F. H. Robertson “saw two birds alight carrying seaweed” on March 11, 1946, when the majority of the adult birds and chicks had already left the gannetry and the breeding season was well over. It would be of interest to know whether this display is the male's prerogative or are perhaps both sexes taking an equal part.
This is a solitary bowing performance of a bird sitting on the nest: the bird rises on its feet, stretches backwards, shakes its head quickly, and then bows three or four times to the right or to the left; the wings are lowered partly extended and held in this position apart from the body. The whole performance, during which a sharp call is uttered, lasts 5 to 7 seconds.
The “solo-dance” ceremony is frequently started by a bird on a nest without egg. This performance seems to be contagious, and several birds sitting nearby, with or without eggs, may follow suit. It is readily started by any disturbance, i.e., the alighting of a bird in the colony, appearance of a human, or for no apparent reason at all. Often the performance is repeated, mostly by birds without eggs, at 3–5 minute intervals, and is followed, in nests with eggs, by a careful rearrangement of egg betwen webs with the assistance of bill. This display still took place frequently on February 4 and on March 11, 1946, when the Rev. F. H. Robertson “saw a number of solo-dances although the breeding season was virtually over.”
This usually gregarious performance seems to fall into the category of activities of the gannets leading to the “mass stimulation” suggested by Fraser Darling.
(iii) “Greeting Ceremony.”
This mutual display of the Australasian Gannet, although in some ways similar for many gannets and boobies, seems to differ from the description of the Blue-footed Booby's (Sula nebouxii) ceremony given by Stonor (1940), and is one in which the emotions of a mated pair of gannets are shown most expressively. It actually starts when a mate, on returning to the nesting site, brakes quickly when a few feet away and, frequently calling, lands sharply alongside the nest. Seldom a mistake is made. If the bird lands on stranger's territory, a fierce fight follows with several birds participating until the interloper regains its own nest area. Immediately after landing, the birds face each other, beaks touching, necks stretched, and calling (Plate 39, Fig. 2). The beaks are then crossed in a tapping action about six times on alternate sides, the sound being audible at 10–12 ft. in spite of noise made by other birds. It would seem that the demonstrating pair need to express, in an audible manner, the ecstatic emotions which seem to permeate both partners. This is followed by waving and dipping of heads and by stroking of upper half of necks on alternate sides about six times. During the whole of this perform-
ance, which lasts up to 10–12 seconds, the birds seem to be completely oblivious of other birds or to nearby man. The tapping of beaks and neck stretching is sometimes repeated. Birds then quieten down, often engaging in an affectionate mutual search for external parasites. At no time does the feeding of a mate occur during this or other displays.
The “greeting, ceremony” as described above may be followed by different actions, depending on the stage of breeding of the pair concerned:
When an egg is present, the bird originally sitting very slowly and carefully shuffles backward off the egg and allows the new arrival to take his or her place with equal care. The latter bird carefully steps on nest and enfolds egg with webs, pushing it in with the help of beak, as it settles down. In the meantime, or within a few minutes, the other bird takes off after stretching its neck for approximately one minute.
When a chick is present in the nest, the new arrival takes the place of the previous guard, which flies off shortly after. The bird on guard may attempt to cover chick up to the full all-white stage; otherwise it will sit near the chick. While on guard it may occupy time in delousing the chick, with older chicks often reciprocating.
When building the nest the “greeting ceremony” is sometimes preceded by the bringing of nesting material described above. When the “greeting ceremony” is over, the bird on the nest arranges the new contribution while the mate flies off for new material. It seems evident that very little material is collected before the egg is laid. Under this heading come also cases when there is neither chick nor egg in the nest; in these numerous cases, after the conclusion of the ceremony the mate settles quietly down on the bare ground.
On rare occasions pairing takes place as a termination of the greeting ceremony. Under these circumstances mating occurs on the nesting place (Plate 40, Fig. 1): the greeting ceremony is short, and then the male catches hold of feathers at the back of the female's head and shakes her rather roughly. He then grabs the whole head in his beak from the rear, and while mounting shakes vigorously. This is accompanied by cackling of the male. Treading and coition occur very rapidly, and one coition only takes place. After coition the display consists of anything from complete disinterest to some bowing and delousing.
It is interesting to note that on February 4, according to Mr. Robertson (in litt.) greeting ceremonies were still frequent. They were “not very common early in the afternoon, but more common as the birds began to return in greater numbers.” Some of these performances seemed to Mr. Robertson “to be rather casual and half-hearted but in other cases more energetic.” Five minutes or so of mutual and, in some cases, of individual preening followed before the birds went to sleep. In one case a resumption of the preening was observed after sleep. During his visit on March 11, 1946, Mr. Robertson reported numerous “solo-dances” and frequent “greeting ceremonies” among members of Colony C. In view of all this and keeping in mind the stage in which this colony was at this time
of the year (Tables 4, 5 and 6), it seems evident that the mutual ceremonies are still performed for a long period after the chick is hatched.
(iv) The Feeding of Chicks and Chicks' Habits.
It is well known that when gannet chicks are about to leave the nesting site and to commence their own life, as in several larger species of sea-birds, they are very fat and usually heavier than the adults. In view of this a heavy demand for food is placed on the parents. This may be a reason why the gannets lay one egg only.
Strangely enough, during our December visit feeding seldom occurred during the hours of daylight. On three occasions only was feeding observed. On the other hand, according to Mr. Robertson (in litt.) during his visit on February 4 “feeding went on continuously throughout the afternoon, becoming more general as the afternoon advanced, when it was possible to see two or three birds feeding at any time.” We are indebted to Mr. Robertson for his description of the feeding display, which has filled up the gaps in our observations and which we give below from his field notebook:
There seems to exist several stages of a fairly well defined display associated with feeding:
The parent bird will alight uttering the cry of the greeting ceremony, and the chick at once drops wings as in the greeting ceremony and points beak upwards, the head swaying from the left to the right as it utters a high-pitched monotonous “cheep”; it was repeated rapidly some four or five times a second. This call appeared frequently to be a prelude to a demand for food from its parent, although sometimes the feeding display begins at stage (b).
The chick stretches out its neck and begins in an attitude suggesting supplication, to attempt to force parent bird's beak apart, first on one side and then on the other. This approach is not pressed at first; but rapidly becomes more insistent, until suddenly, or after two or three half-hearted beginnings the parent bird opens the bill widely.
The chick then inserts its beak within the mouth and throat of parent bird (Plate 40, Fig. 2). The head of the chick almost disappears for a moment, the chick's crown being between the lower and upper mandibles of the parent. Then with a great effort the parent bird regurgitates food, the passing of which it was possible in some cases to observe at times as a thin string of clear, glutinous liquid between the bills. The act of regurgitation lasts for a few seconds only.
Then the chick at once continues with dropped wings and up-pointed beak waving from right to left to utter the monotonous “cheep” referred to earlier. The uttering of this call is easily recognisable by the rapid pulsations of the loose skin of its large black throat. The attitude referred to in this stage of feeding somewhat resembles the attitude of adult birds in the greeting ceremony except for the movement of the beak.
After a time the process begins all over again at stage (b). In most cases this happened two or three times, but in one case as many as 8 were observed.
Stage (d) continued for some time as long as the parent was present, but ceased very soon after the parent departed. Occasionally, to avoid the importunity of the chick, the parent would lay its head on its back in the sleeping position.
Apparently older chicks seem to recognise their parents while still on the wing; in one case a chick advanced about 6ft. between other birds to meet its parent, and feeding followed at once. In other cases two chicks claimed as parent a bird which usually attacked the imposter. In one case, during the first feeding a second chick intervened, and after a tussle the proper chick was fed a second time. Sometimes the sight of an adult bird alighting near a strange chick will induce stage (a) in the chick, but this is not carried on for long, and the adult bird may attack the chick.
If a chick is disturbed by external influences a regurgitation of the food occurs. It was observed on five occasions only, which means that it does not happen as often as it has been described by Oliver (1930). The regurgitating chicks were all at the partly feathered stage, and half to one pound of fish (closely packed herrings) was regurgitated. One chick regurgitated twice within a few seconds. In no case was regurgitation in adult birds observed as a result of a disturbance; this seems to be in striking discrepancy with Oliver's (1930) observation.
The resting habits were observed in chicks of all ages. The chick rests on its body with its head in the direction of the tail of the covering parent. Older chicks sleep on one side of the body in a position which leads even an experienced observer to think they are dead.
Older chicks, from the white downy stage onwards, start to move around. Like adult birds, as soon as they trespass on another bird's territory they are subjected to attacks both from chicks and adults, but no harm resulting from these vicious peckings was observed. Towards the end of the season large congregations of adults and chicks off their nests were observed peacefully sheltering from a strong wind on the slopes of the seaward slope of Colony A. On February 4, on all sides of the rock grown-up chicks were engaged in training for flight: they stretch the head forward, thicken the neck, open and flap the wings three or four times, wag the tail right to left, close the wings, and settle again. In four or five cases the chick rose 6in. or so off the ground and alighted again.
There is little to be added in respect to the general behaviour of adult gannets on the rock to the excellent description given by Gurney (1913) and others.
As with most birds gregariously nesting, the territorial sense is restricted to an individual central area with a radius of approximately 50 centimetres, which seems to form one pair's individual territory. This distance approximates the limits of pecking distance of owners and the neighbouring pairs. Whenever a bird returning to the nest inadvertently alights within a territory of another or when, in taking off from its nest, it is forced by wind conditions or disturbance by humans to infringe on the territory of neighbours,
its presence is violently objected to by all birds concerned. Such objection is demonstrated by pecking, by shrill cries, and by hissing. The invading bird, when departing, normally makes off as rapidly as possible towards the nearest open space, generally the cliff edge, from which it can take off (Plate 40, Fig. 4). If alighting the invader beats its way as quickly as possible to its own territory. On occasions, probably when both birds are conscious of their rights, an individual fight occurs, the antagonists locking beaks and pulling back with great strength. These fights sometimes develop into three-cornered wrangles. The same applies when a man invades the territory, and sometimes the birds may inflict deep cuts. Territorial mistakes, however, are infrequently made by adult birds, which exhibit marked accuracy in locating the nest.
It appears also from our observations that during the daytime a large proportion of the birds is absent from the rock, although there was always a group of birds flying about or roosting on nearby cliffs with a second group resting or playing at sea half a mile away. They were probably the so-called “Barren Gannets” of Gurney (1913)—i.e., non-breeding birds. No diving was observed near the colony. The nearest place where a few birds diving were observed was at Clifton, five miles away.
A few external parasites of the gannets were located and forwarded to Professor Laurence R. Richardson, of Victoria University College. They were identified by Mr. M. Laird as of the genus Philopterus apparently close to P. breviantennatus Piaget.
In order to study some psychological reactions of breeding gannets three experiments with eggs of incubating birds were carried out. All three experiments were started at the same time, one observer being in charge of each experiment. The nests and birds were observed for about 45 minutes, and about six hours later all the nests were carefully inspected, and eventually the eggs returned to their respective owners. All three experiments were conducted on nests and eggs of Colony B, each set being on a different part of the colony.
Experiment I (K. A. W.).
Removing of eggs and placing them at a distance of 4, 8, and 12 in. (10, 20, and 30 centimetres) from centre of nest. The capacity of the Black-headed Gull for rolling displaced eggs back to its nest was studied in the classical experiments of Kirkman (1937). A large number of birds rolled their eggs from 1½ ft. (about 45 cm.) from the centre of the nest. He thinks that 1½ ft. marks the point at which the capacity to perceive the eggs has really ceased. With gannets, however, the situation is different, because owing to a different structure of the nest the eggs do not usually roll out, e.g., when the bird rises off its nest.
4 in. distance: Nests No. 30, 31, and 32. Within 10 minutes all three birds returned to their nests, pushed the egg with the help of the beak into the nest cavity, fetched it with their webs, and resumed incubation.
8 in. distance: Nests No. 33, 34, and 35. All birds came back within ten minutes to their nests. No. 33 and 34 moved the eggs into the nests and quietly resumed incubation. Bird No. 35 returned approximately at the same time as the other birds of this group and settled down on the empty nest without paying attention to the egg. At 6 p.m. all birds were quietly sitting on their nests, No. 35 still sitting on an empty nest.
12 in. distance: Nests No. 36, 37, and 38. All three birds returned to their nest territories within a few minutes, but only bird No. 37 tried immediately to catch its egg, and eventually succeeded in placing it between its webs and resumed incubation. Birds No. 36 and 38 resumed sitting on their empty nests without paying any attention to their eggs outside. At 6 p.m. the situation was unchanged.
Experiment II (J. J. H.).
Placing two eggs in one nest. Six nests with one egg in each were chosen and numbered 21 to 26, and the eggs were moved. Thus nests No. 21, 23, and 25 were empty and No. 22, 24, and 26 contained two eggs each.
The following notes were made on the birds' behaviour: a bird settled on nest No. 23; bird settled on No. 25, but walked immediately over to No. 24; a bird settled on No. 25; a bird walked into nest No. 22, but soon left this nest and walked to No. 21, scratching in the nest and picking bits; “solo-dance” of No. 23; ten minutes after the experiment had started, No. 22 and 26 were still empty; two minutes later a bird settled on No. 26; “solo-dance” of No. 23, 25, and of No. 23 again; 40 minutes after commencement of the experiment a bird settled in No. 22, the last nest not yet occupied since the commencement of the experiment. At 6 p.m. all nests were occupied, but the birds of No. 23 and 25 were standing, while No. 24 held only one egg between webs.
Experiment III (D. E. K. W.).
Placing three eggs in one nest. Nine nests with one egg each were chosen and numbered 11 to 19, and the eggs were moved. Thus the nests No. 11, 12, 14, 15, 17, and 18 were empty, and No. 13, 16, and 19 contained three eggs each.
The following observations were made on the behaviour of this group of birds: birds No. 14 and 15 took off immediately they were disturbed, but both returned within about five minutes and settled on their empty nests, digging about with their beaks as if to locate their eggs; birds No. 12 and 17 remained by their nests while their eggs were removed and settled back as soon as allowed; they also appeared somewhat restless and poked about with their beaks at intervals; birds 11 and 18 also remained by their nests and settled down fairly stolidly as soon as the disturbance ceased; they did not appear to miss their eggs at first; after a few minutes bird No. 15 appeared to accept the state of affairs and settled down; birds 12, 14, and 17 were still restless and poking about after ten minutes; bird 12 was still uneasy after fifteen minutes; No. 11 then became restless for a few minutes; after twenty minutes, however, they had all settled down and were comparatively quiet. Birds No. 13, 16, and 19, who had three eggs each, accepted and covered them immediately with one
foot on either of two of the eggs and the third egg either behind or in front under their breasts just as they happened to be arranged in the nest; however, after 10 minutes they were all trying to re-arrange the eggs. At 6 p.m. birds No. 13, 16, and 19 were still covering three eggs each. Of the others, No. 12 and 15 were sitting, while No. 11, 14, 17, and 18 were standing on their nests. All eggs were then returned to their rightful nests and the birds left to settle down.
Although, in view of the few experiments, it is not possible to draw any far-reaching conclusions, it seems likely that:
The distance of about 8 in. (20 cm.) seems to be the limit within which an egg is still recognised by an incubating bird and rolled back into the nest. It has to be kept in mind that the sex of the bird may play here an important part. An egg lying beyond this distance is no longer recognised by the incubating bird.
From all three experiments it seems evident that the capacity of perceiving the nest is a much stronger factor than the presence of the eggs. All birds returned and stayed on their nest notwithstanding the fact that it was empty. A definite uneasiness, however, was noticeable in the birds whose eggs were removed; but they seemed to be unaware of the presence of additional eggs in their nests.
Finally, the last two experiments seem to prove that though the gannet is actually fitted to incubate one egg only, it may cover a two-egg but not a three-egg clutch.
Discussion and Summary.
The hitherto-known breeding colonies of the gannet (Sula serrator) in New Zealand have been listed, and a suggestion is made that the breeding, distribution, history, and population of the Australasian Gannet should be studied on a national scale by Australian and New Zealand ornithologists. In comparison with the results recently obtained in respect to the North Atlantic Gannet, such studies could give an important insight into several ecological and bird-preservation problems.
The history, the effect of the protection, and the present conditions of the Cape Kidnappers colony are described, and the results of a census are given in Table 3. It brings the total population of all three colonies to 2,837 pairs breeding in the 1945–46 season.
An attempt was made to obtain an insight into the reproduction efficiency of the gannets at Cape Kidnappers, and the conclusion is reached that no more than 16 per cent. of their total number in 1945–46 have been reared to the stage that the chicks are able to leave the rock.
The number and size of the eggs, and the structure and size of the nests have been investigated. It is concluded that there are no more than 0.75 per cent. of nests with two eggs.
Mutual display is prominent in gannets during the breeding season. Bringing of nesting material, the “solo-dance” and the “greeting ceremony” are the main displays associated with breeding. A description of chicks' feeding and other habits of the chick is given.
Three experiments testing the intelligence of breeding birds were arranged and their results are presented.
Fig. 1.—Gannet's Mode of Incubation; egg is being placed between webs of both feet.
(Photo., C.P. McMeekan.)
The opportunity so readily available at Kidnappers, the lack of factual information on record, and the importance of some of the problems raised present a challenge to New Zealand ornithologists to organise a systematic study of this most interesting and beautiful bird—Sula serrator.
We are grateful to Miss D. E. K. Walker and Mr. J. J. Hancock, of Ruakura, for giving their most valuable assistance during our stay at the cape. We wish particularly to thank the Rev. F. H. Robertson, Havelock North, for his help and especially for taking the trouble to visit the gannetry and making some very valuable contributions to this paper. Captain Mark Taylor, R.N., kindly supplied us with some photographs.
We are also grateful to Dr. R. A. Falla, Christchurch, and Mr. R. B. Sibson, Auckland, for supplying their unpublished data of the gannet population in the New Zealand area, and to Mr. K. A. Hindwood, President R.A.O.U., Sydney, for information referring to Australia; to Mr. Allan C. Duff, Honorary Ranger of the Cape Kidnappers Sanctuary Board, for his kind assistance and some information regarding the gannetry. Thanks are also due to the Department of Internal Affairs, Wellington, which supplied us with some interesting historical records, and last, but not least, we are grateful to Dr. W. M. Hamilton for reading the manuscript and supplying his criticism.
|Length. Width.||Length. Width.||Length. Width.||Length. Width.|
|73 × 46||81 × 47||78 × 47||80 × 46|
|78 × 46||83 × 47||78 × 48||83 × 47|
|78 × 49||82 × 49||80 × 47||73 × 45|
|78 × 45||78 × 46||75 × 47||82 × 46|
|75 × 46||73 × 47||75 × 47 Twins||82 × 46|
|79 × 47||73 × 47||78 × 46||77 × 47|
|80 × 49||77 × 47||79 × 46||72 × 46|
|75 × 47||75 × 47||77 × 46||70 × 46|
|75 × 46||74 × 47||75 × 49||81 × 45|
|78 × 47||67 × 46||77 × 48||78 × 49|
|78 × 48||77 × 48||75 × 48||79 × 48|
|75 × 49||80 × 49||79 × 45||71 × 43|
|79 × 48||81 × 47||75 × 47||78 × 46|
|79 × 46||78 × 48||77 × 47||80 × 46|
|75 × 48||79 × 45||75 × 45||78 × 45|
|78 × 50||78 × 47||73 × 47||81 × 45|
|77 × 49||83 × 49||77 × 47||79 × 48|
|76 × 48||80 × 43||76 × 47||74 × 50|
|74 × 47||82 × 48||77 × 48||70 × 45|
|73 × 46||80 × 49||83 × 46||79 × 48|
|75 × 45||75 × 48||88 × 45||75 × 46|
|77 × 45||78 × 48||72 × 47||76 × 49|
|78 × 48||78 × 46||77 × 49||81 × 49|
|80 × 48||82 × 45||84 × 49||83 × 49|
|78 × 48||81 × 47||77 × 47||77 × 49|
|Length. Width.||Length. Width.||Length. Width.||Length. Width.|
|78 × 46||75 × 46||77 × 46||76 × 48|
|80 × 48||79 × 48||78 × 46||77 × 48|
|78 × 45||80 × 48||83 × 46||77 × 48|
|79 × 45||80 × 46||79 × 45||75 × 47|
|82 × 49||78 × 47||74 × 46||81 × 48|
|77 × 49||80 × 47||76 × 46||77 × 48|
|75 × 47||80 × 50||75 × 46||78 × 48|
|77 × 47||75 × 48||75 × 45||78 × 48|
|76 × 45||77 × 50||76 × 46||78 × 45|
|79 × 48||82 × 47||75 × 43||78 × 49|
|75 × 46||77 × 47||85 × 48||79 × 46|
|72 × 47||77 × 47||75 × 48||77 × 45|
|79 × 45||76 × 47|
Andersen, Johannes C., 1908. Maori Life in Aotea. Whitcombe & Tombs.
Buller, Sir Walter Lowry, 1888. History of the Birds of New Zealand. 2nd. Edition, London, Vol. 2, pp. 148 and 179.
Bulletins, Annual Reports, and N.Z. Bird Notes of the N.Z. Ornithological Society, 1939–45. Dunedin.
Edwards, V. C. Wynne-, Lockley, R. M., and Salmon, H. M., 1936. British Birds, 29, pp. 268–276 (not seen by the Authors).
Fisher, James, and Vevers, H. G., 1943. “The Breeding, Distribution, History, and Population of the Atlantic Gannet, Sula bassana. Part I. A History of the Gannet's Colonies and the Census in 1939.” J. of An. Ecol., 12, 2, pp. 173–213.
Gurney, J. H., 1913. The Gannet, a Bird with a History. London.
Huxley, Julian, 1930. Bird-watching and Bird Behaviour. Chatto, London.
Kirkman, F. B., 1937. Bird Behaviour. T. Nelson and Sons, London.
Madden, E. A., 1940. The Grasslands of the North Island of New Zealand. Dep. Sci. and Ind. Res., Bull 79, Wellington.
Marples, B. J., 1946. List of the Birds of New Zealand. N.Z. Bird Notes, Vol. 1, Suppl., Dunedin.
Mathews, Gregory M., and Iredale, Tom, 1921. A Manual of the Birds of Australia. H. F. and G. Witherby, London, pp. 76.
North, Alfred J., 1911. Nests and Eggs of Birds found breeding in Australia or Tasmania. Vol. III. F. W. White, Sydney.
Oliver, W. R. B., 1930. The Birds of New Zealand. Wellington.
Smith, Stuart, 1945. How to Study Birds. Collins, London.
Stonor, C. R., 1940. Courtship and Display among Birds. Country Life Ltd., London.
Witherby, H. F., Jourdain, F. C. R., Ticehurst, N. F., and Tucker, B. W., 1943. The Handbook of British Birds, Vol. IV. H. F. and G. Witherby, London.
Wodzicki, Kazimierz, 1935. Studies on the Stork, Ciconia ciconia L., in Poland. III. The Stork in the Voivodship of Lwow. “Ochrona Przyrody.” Cracow-Warsaw.